Use esbuild without Vite

[kytta]

Instead of battling with Vite (e.g. vitejs/vite#4863) and its configs, I have decided to drop it and to use esbuild directly. Builds are now 6x faster if running the script directly (not via PNPM). This has other advantages:

• we can fine-tune the output of CSS and JS without relying on too many other plugins and dependencies
• we can later re-use the esbuild dev server to build docs (see Automatically build and deploy documentation #102)

[github-actions]

size-limit report 📦

Path	Size
./dist/shareon.min.css	5.97 KB (+0.05% 🔺)
./dist/shareon.es.js	0 B (-100% 🔽)
./dist/shareon.mjs	984 B (+100% 🔺)