Document LANGUAGE extensions in hover and completion

[dawkrish]

Summary

• Add hover and completion documentation for LANGUAGE pragmas.
• Show the one-line description, Since version, Status, Implies, and targeted GHC User Guide link in a Markdown table.
• Link implied extensions to their own guide sections; omit implications for negated extensions.
• Suppress the default HLS hover on LANGUAGE pragma lines so the new documentation is shown once per server.

Verification

• cabal test hls-pragmas-plugin-tests (105 tests passed)
• cabal build exe:haskell-language-server
• git diff --check

This remains a draft for maintainer feedback.