Update Semantic Highlighting and Outline to use Slang@19

[kanej]

Slang@19 comes with bug fixes and runs in WASM, but implies some changes to the API.

Because Slang@19 is ESM, we need to do dynamic imports to use it from the extension's commonjs code:

const { Language } = await import("@nomicfoundation/slang")

This will require reworking the structure of the supporting classes in Semantic Highlighting and Outline.