This ways the client doesn’t need to create a hierarchical structure using the container names. It is also more flexible and allows nesting of test suites + the addition of labels and tags for swift-testing.
The data structure for `TestItem` has been heavily inspired by VS Code’s `TestItem` for the test explorer, which should make it fairly straightforward to integrate these results into the VS Code test explorer.
The naming was quite inconsistent here. Let’s rename these to `LanguageService` to highlight that they belong together.
- ToolchainLanguageServer -> LanguageService
- SwiftLanguageServer -> SwiftLanguageService
- ClangLanguageServerShim -> ClangLanguageService
