fix: Declared mrmime as a runtime dependency (21.x.x)

[Ionaru]

Fixes #100

src/builders/build/builder.ts and src/plugin/index.ts import mrmime at runtime, but the package only declared it in devDependencies, which post-build.mjs strips from the published manifest. The import only resolved while another installed package happened to hoist mrmime (typically the v3 mainline package left in place during migration), and ng build fails with Cannot find package 'mrmime' once that is uninstalled.

This moves mrmime to dependencies, matching the mainline package. Verified locally: pnpm build now emits dist/package.json with mrmime under dependencies, and typecheck, lint, and tests pass.

Note on scope: esbuild and json5 are also imported at runtime without being declared (as are @schematics/angular and @nx/devkit in the schematics and generator entry points). Those are deliberately not added here because they are highly likely to be already installed through Angular's own dependencies (@angular/build, @angular-devkit/*), unlike mrmime, which nothing else in an Angular workspace provides.