Document LanguageName migration and keep the README sample typechecking

[schani]

Fixes #2905.

Problem

Since quicktype-core 23.1.0, the lang option of quicktype() and the parameter of jsonInputForTargetLanguage() are typed as LanguageName | TargetLanguage instead of string | TargetLanguage. The widely-circulated README-style sample that wraps quicktype in a helper taking targetLanguage: string no longer typechecks (TS2345/TS2322), and the resolution — the exported LanguageName type and isLanguageName() type guard — was only discoverable by reading dist/language/types.d.ts. Anyone upgrading from 23.0.x straight to 24.0.0 hits this without any migration guidance.

Approach

No API changes are needed — LanguageName and isLanguageName are already exported from quicktype-core's public entry point. The types stay strict; this PR documents them and pins the docs to the API with a test:

• README: the "Calling quicktype from JavaScript" sample is now TypeScript with targetLanguage: LanguageName parameters (with a note that plain-JS users just drop the annotations), is self-contained/runnable, and gains a snippet showing the escape hatch for dynamic language names: narrow a string with isLanguageName() (which accepts everything quicktype accepts — canonical names, display names, and extensions), with languageNamed() → TargetLanguage instance mentioned as the alternative.
• MIGRATING.md: new "Stricter language-name typing in quicktype-core" section describing when the change landed (23.1.0), the exact error symptoms, and both migration styles; the existing "no public API changes in 24" sentence is qualified so it no longer contradicts what upgraders from ≤23.0.x experience.
• New test test/unit/readme-sample.test.ts: extracts the TypeScript code blocks from that README section and compiles them with tsc --strict against the built quicktype-core, so the sample — and the public exports it relies on — can't silently rot again.

Verification

• npm run build and npm run test:unit pass (8 files, 67 tests, including the new one).
• Negative check: compiling the old targetLanguage: string helper against the built package reproduces exactly the TS2345/TS2322 errors from the issue, confirming the test setup catches the regression class.
• End-to-end: npm packed quicktype-core 24.0.0 into a standalone scratch project, extracted the README snippets verbatim, compiled with tsc --strict, and ran them — the sample emits the expected Swift and Python output.
• No package.json versions touched, per repo convention.

🤖 Generated with Claude Code

[schani]

Closing: we've decided the fuzzy language-name matching (display names / extensions in languageNamed/isLanguageName) reintroduced by #2883 was a mistake, and this PR documents that behavior. #2883 is being reverted instead.