Skwasm heavy (#166619)

This produces a build of Skwasm that works on Firefox and Safari. This
means we use `SkAnimatedImage` for animated gifs and webps and use
builtin ICU data in Skia.

I have unit test suites for Safari and Firefox with dart2wasm and both
`ui` and `engine` test sets. However, there are a few issues with
running these on CI:
* Safari+dart2wasm doesn't work yet until the CI bots are upgraded to
macOS 15, so these have been disabled on CI for now (but you can run the
unit test suite locally).
* Firefox+ui doesn't work because our Linux bots have no GPU and
therefore no WebGL2 support, so that one is disabled. Firefox+dart2wasm
with the `engine` suite is enabled on CI though.

I did make some changes to the host page for our unit test harness so
that Safari actually works though. Even though we're not running on CI,
you can still run locally if you have macOS 15.