docs: document the language-tagged string use case

[jeswr]

Documents the language-tagged string (rdf:langString) use case. Support already exists (ILangString, LiteralAs.langString/langTuple, LiteralFrom.langString/langTuple, Mapping.languageDictionary) and is covered by the existing test suite — this PR only adds the missing documentation. No behaviour change.

Closes #75

What it does

• README: adds a Language-tagged strings subsection under Wrapping Terms with:
  • a turtle snippet ("The librarian"@en) and a getter/setter mapping class using LiteralAs.langString / LiteralFrom.langString ({lang, string} objects), with usage output;
  • a note on the langTuple ([language, value]) variants;
  • a Mapping.languageDictionary example wrapping a one-value-per-language predicate as a JavaScript Map, with usage output.
• JSDoc: adds dense documentation to LiteralAs.langString and LiteralAs.langTuple (summary, @remarks, @throws, titled @example blocks pairing turtle with a mapping class, @see), matching the style of the neighbouring LiteralAs.uInt8Array docs.

Checks

• npx typedoc passes; all new {@link} targets resolve (the only warnings are ones pre-existing on main).
• npm test green (122 tests, 0 failures); npm audit --omit=dev --audit-level=moderate clean.

Review timing: This draft was prepared with Claude; I (@jeswr) will personally review it before it progresses. I'm currently batching a lot of work in flight, so expect active review Wednesday-Friday (8-10 July).