Add LSP semantic token support for Power Query

[JordanBoltonMN]

Enables semantic highlighting by implementing the standard LSP textDocument/semanticTokens/full protocol, replacing
the previously scaffolded but disabled custom request approach.

Changes:

• server/src/semanticTokens.ts (new) — Token legend and delta-encoding logic, extracted into its own module.
  Encodes PartialSemanticToken[] from PQLS into the LSP wire format.
• server/src/server.ts — Advertises semanticTokensProvider capability and handles textDocument/semanticTokens/full
  via connection.languages.semanticTokens.on(...). Removed commented-out custom powerquery/semanticTokens handler.
• client/src/extension.ts — Removed commented-out client-side provider registration (vscode-languageclient handles
  this automatically when the server advertises the capability).
• package.json — Added semanticTokenScopes mapping 14 token types to TextMate scopes for theme coloring.
• server/src/test/semanticTokens.test.ts (new) — Tests covering legend correctness, real PQLS analysis output for
  all emitted token types (variable, function, parameter, keyword, number, string, type, operator), and encoding
  invariants (monotonic positions, legend bounds, immutability).

Token legend: 13 types (variable, function, parameter, property, keyword, number, string, comment, type, namespace,
operator, typeParameter, modifier) and 5 modifiers (declaration, definition, readonly, defaultLibrary, deprecated).