Implement completion and snippet management using builtins

[cvigilv]

Refer to:

• https://github.com/neovim/neovim/pull/25301
• https://github.com/neovim/neovim/pull/27339
• :h vim.lsp.buf.completion()
• :h vim.snippet

[cvigilv]

Tested it, cmp is better (for now). Closing until builtins are good enough.