[lsp-server] Add support for custom regions via comments

## Current Behavior (if applicable)
``` gql
query {
foo {
bar
baz
# region
zim
zam
zop
# endregion
}
}
```

The region/endregion comments do not activate custom code folding regions.

## Desired Behavior
`# region` and `# endregion` or some variant like `# :region` and `# :endregion` should allow for custom code folding regions. This is especially useful for large queries within one resolver.

This specific syntax is an existing [feature](https://code.visualstudio.com/docs/editor/codebasics#_folding) for other languages in VS Code and there are other mechanisms for other editors. Regardless of the actual implementation, first-party support for some sort of custom folding would be greatly appreciated!