When a new preference is enabled, Go To Definition shows definitions from Go To Source Definition when available, instead of its typical results.

This is achievable without TS Server changes, allowing it to work on TypeScript versions as old as 4.7, but perhaps not optimally so. As it is now, definitionAndBoundSpan is still called first, because findSourceDefinition does not return the text span used to draw the underline, and it also does not return fallback results to type declarations, which I think we would want if Go To Definition is fully replaced. findSourceDefinition is called subsequently, and the definitions it returns replace the ones returned by definitionAndBoundSpan. If findSourceDefinition returns no results, the response from definitionAndBoundSpan is used as a fallback.

definitionAndBoundSpan is typically very cheap, so this might be acceptable. But there are two ways we could improve performance:

1. Make a new TS Server command that returns source definitions, along with the text span, and falls back to type declarations when no other definitions are found. This would allow VS Code to trigger only one request, provided the TS version is new enough to have this capability.
2. A bigger change to VS Code, but the best thing for perceived performance would be to support separate requests for drawing the underline and showing the definitions. findSourceDefinition can be considerably more expensive than definitionAndBoundSpan, so it would be nice to trigger the latter when hovering with ctrl/cmd, and then trigger the former once the user clicks.