Feature Request: Improve VSCode plugin and publish it in the Marketplace

Implement the same (or most of) [features that IntelliJ have](https://pkl-lang.org/intellij/current/highlights.html)

Start by reviewing the existing VSCode plugin and the IntelliJ feature list linked in the issue. Compare the current VSCode capabilities with those features, then define a concrete scope and acceptance criteria for the Marketplace publication; the issue does not name files, tests, or specific features.

Written by the indexing model from the issue text.