[vscode] Support FoldingRangeProvider#onDidChangeFoldingRanges events

[planger]

Feature Description:

Support plugins contributing a FoldingRangeProvider to optionally signal via onDidChangeFoldingRanges that the folding ranges from this provider have changed, see https://code.visualstudio.com/api/references/vscode-api#FoldingRangeProvider

[JonasHelming]

we will have a look at this

[ndoschek]

Hi @Gozzim!
Thanks for your PR! To test this, we could have a look at the existing TestFoldingRangeProvider unit tests in vscode.
We could add a similar test to Theia and add the newly added optional event to the TestFoldingRangeProvider class.
Then we could test the event behavior in a similar manner as in other unit tests, e.g. in debug.test.ts or terminal.test.ts.

Alternatively, you could create and register a test implementation for a FoldingRangeProvider (or extend an existing one) because as far as I can see, onDidChangeFoldingRanges is not used anywhere yet.
With this test implementation a message notification could be fired if the folding ranges change for example.
Then you could provide the test implementation code in the PR for the tester/reviewer.

If possible however, I would go for adding a test for it. If I can help you out with something, just ping me again :)