Fix headlines.nvim plugin with markdown config

- Uses this fix:
  - lukas-reineke/headlines.nvim#41
- Added to `lua/plugins/headline.lua` new config overrider
- Full spec from lazyvim inclusion is here:
  - https://www.lazyvim.org/extras/lang/markdown#headlinesnvim

lua/plugins/headline.lua

@@ -0,0 +1,29 @@
+-- Fix taken from:
+-- https://github.com/lukas-reineke/headlines.nvim/issues/41
+return {
+  "lukas-reineke/headlines.nvim",
+  dependencies = "nvim-treesitter/nvim-treesitter",
+  opts = {
+    markdown = {
+      fat_headline_lower_string = "▔",
+    },
+  },
+}
+-- Full Spec taken from lazyvim docs:
+-- https://www.lazyvim.org/extras/lang/markdown#headlinesnvim
+-- opts = function()
+--   local opts = {}
+--   for _, ft in ipairs({ "markdown", "norg", "rmd", "org" }) do
+--     opts[ft] = {
+--       headline_highlights = {},
+--       -- disable bullets for now. See https://github.com/lukas-reineke/headlines.nvim/issues/66
+--       bullets = {},
+--     }
+--     for i = 1, 6 do
+--       local hl = "Headline" .. i
+--       vim.api.nvim_set_hl(0, hl, { link = "Headline", default = true })
+--       table.insert(opts[ft].headline_highlights, hl)
+--     end
+--   end
+--   return opts
+-- end