Improve user experience wrt transparent backgrounds

[Jint-lzxy]

Feature description

This is part of #627 (UI update). The issue is if the user supplied non-zero values to pumblend (and winblend), several plugins' popup windows would be poorly configured - all having completely black backgrounds as "they have nothing to blend with" 🤣 The plan is to list all problematic plugins here and open a PR for this.

Additional information

List of related plugins:

• *specs.nvim
• *nvim-notify
• *nvim-scrollview (++might be hard to see under current spec)
• wilder.nvim
• nvim-treesitter-context
• nvim-cmp
• mason.nvim
• lspsaga.nvim
• lsp_signature.nvim

NOTE: For plugins with asterisks (*) see below for an explanation.

Alternatives considered

We can also warn users who use transparent backgrounds not to change pumblend or winblend, rather than commenting them out. But plugins like nvim-notify and specs.nvim (with asterisks in their names) that use Normal (or other hl groups with no bg set) as their default highlight still need to be updated b/c otherwise they will use #000000 (fallback) as highlight.

Miscellaneous

At the same time, we can also update nvim[_buf|_win]_[gs]et_option API calls as AFAIK they're deprecated in NVIM v0.10.