This change implements a new "scheduler" for WebAssembly using binaryen's asyncify transform.
This is more reliable than the current "coroutines" transform, and works with non-Go code in the call stack.
runtime (js/wasm): handle scheduler nesting
If WASM calls into JS which calls back into WASM, it is possible for the scheduler to nest.
The event from the callback must be handled immediately, so the task cannot simply be deferred to the outer scheduler.
This creates a minimal scheduler loop which is used to handle such nesting.
Previously the cache would be stale for every new branch.
With this change, PRs use the cache from the base branch and therefore
don't need to rebuild LLVM from scratch.
GitHub Actions is faster and much better integrated into GitHub than
Azure Pipelines, and is in general easier to use. Therefore, switch to
GitHub Actions for our Windows builds and tests.
