More powerful folding ranges API

When VS Code folds the range, it just completely hides its contents:

When IntelliJ folds the range, it shows a certain replacement string instead:

The latter behavior is more flexible, and it would be cool if the protocol supported it. It allows for the following features:

• folding a subrange in the string, which allows to hide boilerplate (see this post for example)

• folded blocks take one line instead of two:

• showing some important info from the fold, without making it more verbose:

To implement these features, we need to add two things to the protocol:

• a replacementText?: string field to FoldingRange (this covers bullet points 1 & 2)
• add a group?: number field to FoldingRange, such that all ranges in the group are folded-unfolded in lockstep. This covers use case 3, where { and } are separate folding ranges, linked into one group

However, I assume that the bulk of work here needs to happen on the editor's side as it seems like VS Code doesn't support such replacements yet.