automatically load css at runtime as fallback

[mayank99]

Changes

1. In styles.css, we will set a variable --_iui-v3-loaded. At runtime, we check for this variable and do nothing else if found.
2. If variable not found, that means styles are not loaded. We will first attempt to use a side-effect import to dynamically import styles.css. Many bundlers know how to resolve this, so we should be good.
3. If this doesn't work, we'll fall back to loading styles.css from a CDN.

All of this only happens when using theme="inherit". The includeCss prop can be used to control this behavior for other themes as well.

Testing

Tested thoroughly across different bundlers/frameworks.

• Vite and Astro works ok.
• Nextjs complains about global CSS within components, even though the code is not even called. Can be worked around using transpilePackages.
• Released 3.2.0-dev.0 for external testing.
  • Tested in sandboxes:
    • ✅ vite
    • ✅ webpack
    • ✅ nextjs
    • ✅ cra
    • ✅ esbuild
    • ❌ remix
  • Tested the "v2 consuming v3" scenario externally: Adjust to iTwinUI V3 breaks. admin-components-react#113 (comment)

Docs

Added jsdocs and changeset.

[mayank99]

i've retested this PR and #1744 via 3.2.0-dev.0, works fine everywhere except remix. going to merge and release this now 🚀