[Folding ranges]: support more AST nodes

[tatchi]

I'm wondering if we couldn't improve the result of these tests to be more visual. We could try indicates the locations directly on the code block itself.

So given the following code:

    match 
      Some 
        "large expr" 
    with 
    | None -> 
      () 
    | Some _ -> 
      print_endline "foo";
      print_endline "bar"

With the produced locations:

      Array [
        Object {
          "endCharacter": 21,
          "endLine": 8,
          "kind": "region",
          "startCharacter": 0,
          "startLine": 0,
        },
        Object {
          "endCharacter": 4,
          "endLine": 5,
          "kind": "region",
          "startCharacter": 6,
          "startLine": 4,
        },
        Object {
          "endCharacter": 21,
          "endLine": 8,
          "kind": "region",
          "startCharacter": 8,
          "startLine": 6,
        },
      ]

We could produce:

>   match 
      Some 
        "large expr" 
    with 
>   | None -> 
      () 
>   | Some _ -> 
      print_endline "foo";
      print_endline "bar"

The problem is that we only see the startLine but not the endLine.

Another option might be to copy what vscode does: https://github.com/microsoft/vscode/blob/main/extensions/html-language-features/server/src/test/folding.test.ts#L42

It doesn't use snapshot testing but manually assert on what should be the {start,end}Line. A comment with the line number is also added to the code snippet.

@rgrinberg any thoughts ?

[tatchi]

I tried to identify more use cases where folding would make sense (together with some tests to cover them). Hopefully it does make sense.

[rgrinberg]

@ulugbekna could you take over reviewing this PR? I don't have time to look closely

[ulugbekna]

@rgrinberg sure

[ulugbekna]

LGTM, thanks @tatchi ! :-)

[ulugbekna]

Ready to be merged as soon as CI runs

re: more visual tests

I'm all for it. Maybe we could have something like this, ie we use IDs to see start and end of folds:

>1     match 
           Some  "large expr" 
         with 
>2     | None -> 
<2       () 
>3     | Some _ -> 
            print_endline "foo";
<1 <3   print_endline "bar"

What vscode does is also cool

[rgrinberg]

Can we squash the commits before mergin?