Add support for awk-language-server linter

**Name:** awk-language-server
**URL:** https://github.com/Beaglefoot/awk-language-server

Excerpt from repo:

> Implementation of AWK Language Server based on [tree-sitter](https://github.com/tree-sitter/tree-sitter) and [tree-sitter-awk](https://github.com/Beaglefoot/tree-sitter-awk).

Temporary workaround (until ALE integration is implemented & released): users can define a custom lsp/linter in their `~/.vimrc` file, like so:

```viml
call ale#linter#Define('awk', {
\ 'name': 'awk-language-server',
\ 'lsp': 'stdio',
\ 'executable': 'awk-language-server',
\ 'command': '%e',
\ 'project_root': { _ -> expand('%p:h') }
\})
```