fix: handle asynchronous minification and HTML emission errors

[OskarEichler]

Fixes

• Await HTML minification so async parser failures receive the existing actionable error message (including minify: false). The synchronous catch currently misses rejections from html-minifier-terser.
• Forward late HTML-emission rejections to Webpack's completion callback. Errors during filename interpolation or asset emission currently leave that callback uncalled and cause an unhandled rejection.

postProcessHtml remains Promise-returning; making it async also removes redundant Promise wrappers. Successful HTML output, plugin options, and supported Node versions are unchanged. No breaking API changes or dependency changes.

Verification

• All 161 existing tests across four suites pass.
• JavaScript lint, formatting, spelling, and git diff --check pass.
• Sixteen successful-output comparisons against unmodified upstream cover production/development and disabled/default/custom minification.
• Inline checks cover real asynchronous parse failures, invalid HTML input, unchanged synchronous/asynchronous non-parser errors, and real Webpack compilations with injected filename/asset-emission failures. The completion callback now fires exactly once with the error.
• Plain tsc reports existing TS2802 on the unrelated Set spread in getAssetFiles; reproduced against unmodified upstream using the same compiler host. tsc --downlevelIteration passes with this patch. No TypeScript configuration changes are included.
• No test files were added or changed. The full CI OS/Node-version matrix was not run locally.