Build Web SDK in wasm build tree

[eyebrowsoffire]

This makes some improvements to the web build process.

• felt test no longer requires any build folder at all, unless --use-local-canvaskit is specified. To compile tests, it now uses whatever dart sdk that felt.dart is invoked with (which is by default the prebuilt dart sdk)
• felt build only invokes the --wasm --runtime-mode=release of the gn build, and that build now produces the flutter_web_sdk and the .zip artifact. This build also no longer relies on building or copying the dart sdk to the output folder, it just uses the prebuilt sdk where it is.
• We don't produce a random unused xcodeproj in the wasm build anymore

We need to do some followup recipe changes to take full advantage of this, which I've outlined here: https://github.com/flutter/flutter/issues/113303

The ultimate goal is going to be:

1. The engine build produces a single flutter-web-sdk.zip artifact that rolls into the framework, and no longer has platform specific artifacts.
2. The flutter tool consumes the flutter-web-sdk.zip artifact instead of the platform specific artifacts
3. The host build no longer builds the flutter_web_sdk targets at all
4. We don't do host builds at all to do any web engine CI stuff.

[flutter-dashboard[bot]]

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