feat: Add support for wasm compilation

[scottmarchant]

Change summary

DRAFT ONLY

Adds support for compiling RxSwift to wasm using the Swift for WebAssembly SDK.

Change details

• Several preprocessor statements used to enable wasm compilation
• Added new swift-atomics and swift-dispatch-async dependencies for WASM builds only

Outstanding tasks

• Determine why existing CI pipelines fail and fix any outstanding problems.
• Evaluate the hack and todo items in this diff. Consider possible courses of action or whether the tradeoffs are acceptable for now
• Add CI to ensure wasm builds keep compiling with future changes

Testing Done

Tested compiling and running this as part of a large web app.

Note that this PR is part of a larger effort by PassiveLogic to improve Swift for WebAssembly support