[Neovim] Implement filepath completion for better navigation

I made [`filepaths-ls.nvim`](https://github.com/antonk52/filepaths_ls.nvim) - in memory LSP for filepath completion. This was a missing piece for me to to switch to a more native mini.completion plugin from blink/nvim-cmp Over a year ago I built a similar [basics_ls](https://github.com/antonk52/basics-language-server/) nodejs language server and [shared](https://www.reddit.com/r/neovim/comments/1g4dnry/basics_ls_lsp_for_buffer_path_and_snippet/) it on this subreddit. filepaths_ls is a more capable Lua port of the filepath completion part of that server does not need nodejs. Buffer words and snippets were not ported because `mini.completion` already covers those well enough for me. I started working on it because built-in path completion `<C-x><C-f>` still feels awkward for me due to paths resolved from cwd and not current buffer by default, inserted expanded env variables without an option to opt out. It is still non-trivial to get built-in filepath completion to resolve relative to the current buffer and it comes with some caveats. Once `mini.nvim` added command-line completion, path completion was the missing piece for me to switch to `mini.completion`. I like the overall approach, especially LSP completion with words fallback. `mini.completion` is basically enhanced built-in completion, so after adding this project my config is now mostly `mini.nvim` plus four other plugins. I've used it for the past few weeks and happy with the result. If you want to use the native completion instead of `blink.cmp` or `nvim-cmp`, or if you already use `mini.completion`, you may find it useful too. Repo: https://github.com/antonk52/filepaths_ls.nvim