feat: expose the extractor as a library

[korsvanloon]

The package ships dist/index.js today — tsdown already builds src/index.ts as an entry — but declares only a bin, so nothing can import it. This makes the extractor usable as a library.

Motivation: Evolve's storefront needs to combine extracted labels from a project's own source with the default messages its installed feature packages ship, then decide the output itself. That needs the extraction step without the read-merge-write around it.

Changes

exports map. Points at the already-built dist/index.js / dist/index.d.ts.

buildLabels({ input, source, fallback, onFile }) returns the label tree and writes nothing:

import { buildLabels } from "@labdigital/intl-extractor";

const labels = await buildLabels({
  input: "./src",
  source: existingLabels,
  fallback: (path) => `[${path.at(-1)}]`,
});

processFiles is now the read-merge-write wrapper around it, unchanged in behaviour.

A fallback option for a label with no entry in the source. It receives the full key path (["Cart", "nested", "deep"]) and defaults to the last segment — exactly what was hardcoded before, so existing output is unchanged. We want [label] rather than label, because a key like free has free as a plausible English translation and nothing could then tell a finished string from an untouched one.

onFile so the library stays silent and the CLI keeps its per-file logging.

Two fixes, easy to drop if you'd rather they were separate

• processFiles did not await its writeFile, so the process could exit before the write flushed.
• Its if (!sourceFile) "no existing source file found" branch was unreachable — readFile throws on a missing file, so the CLI errored instead of building from scratch. Now it reads through a try, and an empty file is treated as no labels rather than a JSON parse error.

Testing

Existing snapshot tests pass unchanged, which is the evidence the refactor preserved behaviour. Added coverage for the default fallback, a custom fallback and the paths it receives, source values winning over the fallback, and buildLabels returning without writing.

18 tests pass; pnpm check is clean.