Make languageNamed accept display names and extensions again

[schani]

Problem

languageNamed() and isLanguageName() in quicktype-core stopped accepting display names ("TypeScript", "C++", "JSON Schema") and file extensions ("kt"), and started throwing Unknown language name: ... instead of returning undefined for unknown names.

Most visible fallout: app.quicktype.io stores the chosen language's display name in localStorage and calls languageNamed() with it on load, so returning users got a blank page until they cleared localStorage.

Root cause

The v23.1.0 rewrite (da1238c) of packages/quicktype-core/src/language/All.ts changed languageNamed() to match only the lowercase entries of each language's names array and to throw on a miss. Before that, it matched names, displayName, and extension, and returned undefined when nothing matched.

Fix

Restore the old matching behavior in All.ts:

• match names, then displayName, then extension, all case-insensitively;
• names take precedence over extensions, so "js" still resolves to JavaScript rather than Flow (whose extension is "js");
• return undefined for unknown names instead of throwing (all in-repo callers already guard with isLanguageName() or defined(), and getTargetLanguage() in Run.ts still reports DriverUnknownOutputLanguage for unknown names);
• keep the precise LanguageNameMap[Name] return type via an overload for calls with a known LanguageName literal, while plain strings get TargetLanguage | undefined;
• isLanguageName() now delegates to languageNamed(), so it accepts the same spellings.

Tests

The regression test was written first and demonstrated failing before the fix, on top of current master:

• test/check-language-named.ts (wired into test/test.ts alongside the existing checkJavaEnumAcronymCasing guard) asserts that languageNamed()/isLanguageName() resolve lowercase names, display names, and extensions case-insensitively, and return undefined/false — not throw — for unknown names. The fixture harness only ever passes exact lowercase names, so no fixture input can catch this; asserting on the API directly is the right slot.
• Against unfixed master the check fails: it doesn't even compile under the harness tsconfig (the old signature rejects plain strings), and with ts-node --transpile-only it reports 16 runtime failures, e.g. languageNamed("TypeScript"): expected TypeScript, got throw: Error: Unknown language name: TypeScript.
• After the fix it passes (type-checked, no --transpile-only).

Verification

• Runtime repro from the triage: require("quicktype-core").languageNamed("TypeScript") threw before, now returns the TypeScript language; languageNamed("nope") returns undefined.
• CLI: node dist/index.js --lang TypeScript sample.json now works; --lang nonexistent still errors with Unknown output language nonexistent.
• FIXTURE=typescript-zod script/test passes (210 tests), which also runs the new check.
• packages/quicktype-vscode type-checks cleanly against the new signatures (tsc --noEmit).
• Not verified locally: the app.quicktype.io web app itself (its source is not in this repo), and non-installed language toolchains (php, dart, dotnet, java, kotlin) — but this change touches no code generation, only language-name lookup, and CI runs the full matrix.

Fixes #2769

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