feat(umd): add UMD bundle for CDN/<script> use (src/index.ts)

[paulpach]

Summary

Follow-up to #75 and the suggestion in #95's closing comment.

Adds a UMD bundle targeted at src/index.ts (the existing public entry point) so CDN / <script> consumers who don't run a bundler can use wiremd without any build tooling.

What changed

File	Change
vite.config.browser.ts	New config — UMD-only build from src/index.ts, all deps inlined, minified with terser
package.json	Two new scripts: build:umd and dev:umd

No new subpath export (wiremd/browser) is added — that's intentional. The UMD file lives in dist/wiremd.umd.js and is already covered by the existing "files": ["dist"] entry.

Output

dist/wiremd.umd.js  228 kB │ gzip: 53 kB

Usage (CDN)

<script src="https://unpkg.com/wiremd/dist/wiremd.umd.js"></script>
<script>
  const ast = wiremd.parse('# Hello\n[Button]');
  const html = wiremd.renderToHTML(ast);
  document.body.innerHTML = html;
</script>

Notes

• Does not include the CLI (src/cli/) — src/index.ts only re-exports parser + renderer core
• No Node.js built-ins in the bundle (parser/renderer core is pure JS)
• This PR is intentionally minimal — the ES module / bundler story is handled by fix(build): externalize deps so browser bundlers can consume wiremd #95

[paulpach]

@akonan as you requested