Add autocomplete example

It looks like `vim-ale` supports autocomplete.

I'd be interested to see what that could look like.

Could probably prototype with vscode-language-server (but it seems like it's wicked slow, so maybe not production?).

```text
let g:ale_completion_enabled = 1
let g:ale_lsp_suggestions = 1
```

```text
~/.vim/pack/plugins/start/ale/
doc/ale-supported-languages-and-tools.txt
autoload/ale/completion.vim
autoload/ale/lsp.vim
plugin/ale.vim
```

Start by reading doc/ale-supported-languages-and-tools.txt, autoload/ale/completion.vim, autoload/ale/lsp.vim, and plugin/ale.vim to understand the existing autocomplete and LSP entry points. Prototype the example with vscode-language-server as suggested, and consider its performance; done means the repository contains a usable autocomplete example and its behavior is demonstrated.

Written by the indexing model from the issue text.