x/tools/gopls: multiline strings should be foldable in the editor

[matskiv]

Is your feature request related to a problem? Please describe.
Some multiline strings needlessly take up space in the editor and it is not possible to fold them into a single line.

Describe the solution you'd like
Every multiline string should become a foldable region.

Describe alternatives you've considered
I quickly checked available extensions, but there doesn't seem to be one for this exact use case.

Additional context

[findleyr]

This is a good suggestion, and I think it's a good candidate for external contribution.

Marking as help-wanted, though @ansaba might be interested in taking this.

[natescode]

I'd like to try to take this one on. It'd be my first contribution (already followed the contribution guide).

What's my next step?

[hyangah]

The relevant code place is https://go.googlesource.com/tools/+/refs/heads/master/internal/lsp/source/folding_range.go

For tests, modifying the existing source_test.go and add test data for the new case here will be sufficient.

And, please check the instruction for gopls contributors.

cc @suzmue @findleyr

[rhnvrm]

Hey

I worked on implementing this at golang/tools#354