Document how to test/develop the VSCode extension

Hi!

When trying to add a feature to the VSCode extension (and language server), I had a hard time figuring out how to set things up and actually test my changes. Would probably be nice to document this somehow. Admittedly I'm not an experienced VSCode user or dev, so things I got stuck on might be obvious to others, but I still think some form of basic guide would be very nice.

No specific files or tests are named; first locate the VSCode extension and language-server entry points and try the current development and testing setup. Document the prerequisites, setup steps, and test workflow so a contributor can develop and verify a change without guesswork.

Written by the indexing model from the issue text.