feat!: large rework of configuration logic

[stevearc]

This PR bundles several changes related to how conform can be configured. There are two breaking changes in the form of deprecating a method and deprecating the formatter alternation syntax, however both will continue to work for now after one warning notification. The final breaking change is dropping support for Neovim 0.8, which was done because the tests now fail on 0.8 due to nvim-treesitter. Since 0.9 has been around for a while, it's easier to just bump the requirement and bundle that breaking change with these others.

Default format options

feature: New default_format_opts option that will allow you to configure the default behavior of conform.format()
motivation: The primary reason for this is because the per-filetype configuration (below) needs this to make sense. If we can configure the lsp_format on a per-filetype basis, we also probably want to be able to customize the default value to avoid repetition.
example:

require("conform").setup({
  default_format_opts = {
    lsp_format = "fallback",
    timeout_ms = 500,
  },
})

Per-filetype configuration of format options

feature: formatters_by_ft can now customize some format options
motivation: make it easier to customize behavior for specific filetypes. Previous advice for changing the LSP fallback behavior for one filetype was fairly involved. People frequently perform formatting in two places: on save, and on a particular keymap (e.g. <leader>f). It can be annoying to keep the behavior in those two places in sync. This, combined with default_format_opts, should make it much easier to keep the logic simple in both places.
example:

require("conform").setup({
  formatters_by_ft = {
    go = { "goimports", "gofmt", lsp_format = "never" },
    rust = { "rustfmt", lsp_format = "fallback" },
    javascript = { "prettierd", "prettier", timeout_ms = 2000, stop_after_first = true },
  },
  format_on_save = {},
})
vim.keymap.set("", "<leader>f", function()
  require("conform").format({ async = true })
end, { desc = "Format code" })

Notification for no available formatters

feature: You will now get a once-per-filetype notification when you call conform.format on a buffer that does have formatters configured when none of those formatters are available (i.e. they aren't installed). There is an option to disable this notification.
motivation: #480
example:

require("conform").setup({
  -- to disable these new notifications
  notify_no_formatters = false,
})

Deprecate formatter alternation syntax for option to only run the first available formatter

feature: Previously you could use nested list syntax like { { "prettierd", "prettier" } } to run the first formatter available instead of all formatters. Now, conform.format takes a single boolean option stop_after_first that will determine whether to use all formatters, or just the first available.
motivation: This is a useful feature, but the syntax is a weird special case and complicates the data structures we accept and deal with internally. It's also harder for new users to understand what magic is happening.
example:

require("conform").setup({
  formatters_by_ft = {
    -- deprecated syntax
    -- javascript = { { "prettierd", "prettier" } },
    javascript = { "prettierd", "prettier", stop_after_first = true },
  },
})

Note that we are aware that this new format is slightly less flexible, since you can't use constructs like { { "prettierd", "prettier" }, "injected" } anymore. This behavior is still possible, you will just need to write it yourself using get_formatter_info and checking what is available. We think that the simplification of the code and the common case is worth the added complication for these advanced uses.

Deprecate will_fallback_lsp for list_formatters_to_run

feature: new API method that will return the exact list of formatters that will be run, accounting for all stop_after_first, LSP fallback, and other logic.
motivation: the original purpose for will_fallback_lsp was for use in statusline plugins to help them determine if the formatters will be used or if conform will use LSP instead. With the refactor of lsp_format to add more ways LSP formatting can work, it makes more sense to just bundle all this logic up into a function that just gives you what you want. This new function should make it easier to integrate conform into statuslines.
example:

_G.conform_status = function()
  local formatters, use_lsp = require("conform").list_formatters_to_run()
  local names = vim.tbl_map(function(formatter)
    return formatter.name
  end, formatters)
  if use_lsp then
    table.insert(names, "LSP")
  end
  return "[" .. table.concat(names, ", ") .. "]"
end

vim.o.statusline = [[%!v:lua.conform_status()]]

[lukoshkin]

  json = { { "prettierd", "prettier" }, "fixjson" },
  markdown = { { "prettierd", "prettier" }, "markdownlint" },

How to implement these settings with the new logic? That is, I want to use fixjson and markdownlint always and make it only optional and first available in the pair "prettierd", "prettier"

[stevearc]

@lukoshkin there is a recipe here https://github.com/stevearc/conform.nvim/blob/master/doc/recipes.md#run-the-first-available-formatter-followed-by-more-formatters