Configures the flutter/packages CI to run web tests using the `canvaskit` renderer, which is the new default for web apps.
It also does some minor changes:
* Skips tests on the web for `package:palette_generator` (cleanup issue created below)
* Makes the `palette_generator` example run on the web. This was used to manually verify that the package still works as expected with canvaskit.
* Tweaks the README of the `pointer_interceptor` example apps, so they match what's happening in there.
## Issues
* Fixes https://github.com/flutter/flutter/issues/143543
Co-authored-by: David Iglesias <[email protected]>