vim plugin specific snippets

[teto]

Issue description

Some of the vim plugins need a specific vim configuration to work on nixos, we should add these to nixpkgs (via a passthru key) so that either
1/ users can reference these snippets e.g. in a home manager configuration:

      programs.neovim.plugins = [ {
        plugin = unicode-vim;
        config = pkgs.vimPlugins.unicode-vim.advisedSnippet; 
        # the advisedSnippet being: 
        # with "let g:Unicode_data_directory='${pkgs.vimPlugins.unicode-vim}/share/vim-plugins/unicode-vim/autoload/unicode'
      }

or
2/ the neovim wrapper can add these snippets to the init.vim / init.lua

The alternative of patching the plugin doesnt suit me as it makes bug reports harder and in case we dont patch correctly could taint the nixos reputation: it's best to generate a config that the user can then report to the vim plugin author.

An additionnal issue is that some plugins depend on treesitter grammars such as rest.nvim: without the grammer it triggers
no parser for 'http' language, see :help treesitter-parsers so we should be able to set that as a dependency

Here I want to collect such snippets until we agree on the proposition/naming:

• unicode.vim needs:

let g:Unicode_data_directory='${pkgs.vimPlugins.unicode-vim}/autoload/unicode'

• coq_nvim needs (see vimPlugins.coq_nvim needs aditional handling #168928):

g.coq_settings.xdg

• vim-markdown-composer can now set the path to the binary which means we wouldn't have to patch it anymore configure path to binary (or ignore it) euclio/vim-markdown-composer#69 (comment)

• sql-nvim; let g:sql_clib_path = '${pkgs.sqlite.out}/lib/libsqlite3.so'

• fzf-hoogle-nvim needs vim.g.hoogle_fzf_cache_file = vim.fn.stdpath('cache')..'/hoogle_cache.json'

• vim-grammalecte let g:grammalecte_cli_py='${pkgs.python3Packages.grammalecte}/bin/grammalecte-cli.py' (see vimPlugins.vim-Grammalecte: init at 2021-07-27 #151095)