This request is generally useful, not only for tests within SourceKit-LSP but also:
- In editor tests that want to test the integration with SourceKit-LSP
- In code analysis tools that want to gather project information using SourceKit-LSP and need an up-to-date index for that.
Remove the experimental feature guard from `workspace/_synchronize`, consequently rename it to `workspace/synchronize` and only guard the `buildServerUpdates` option on the synchronize request by an experimental feature because its long-term usefulness is still not fully understood yet.
