Dev server experience could be improved

[mayank99]

Currently we rely on asking every contributor to use a vscode extension like Live Preview to manually open the minified HTML file. This is a tedious and error-prone process (might forget to run build:watch or might open the unminified HTML file). Plus the auto-reload experience feels really messy, with the extension reloading multiple times.

It would be nice to offer a command like yarn dev that automatically serves a proper reliable dev server, similar to the vite playground in iTwinUI-react.

[gretanausedaite]

This also could be useful in using npm packages in css demo pages (eg. fontsource for fonts).