Add a Wasm workflow

[kkebo]

Description:

Swift now officially supports WebAssembly (Wasm), so it's a good time to add a workflow for Wasm.

context: apple/swift-argument-parser#794 (comment)

Things to consider:

• What jobs should we add?
  • Test targets often have dependencies that are difficult to build for Wasm, such as Foundation and Dispatch, and may not be able to build, so a job that simply builds without testing is important.
  • The ability to filter targets in order to build only the library portion is also important. (like swift build --target <target name>)
• What Swift SDKs should we use?
  • Swift SDK for WebAssembly on swift.org
    • swift-<version>_wasm
    • swift-<version>_wasm-embedded
  • SwiftWasm
    • <version>-wasm32-unknown-wasi
    • <version>-wasm32-unknown-wasi-embedded
    • <version>-wasm32-unknown-wasip1-threads
    • <version>-wasm32-unknown-wasip1-threads-embedded
• Which Swift versions should we support?
  • Some versions may lack support of some core libraries.

Related issues:

• Add option to build with the Static Linux SDK #34 (This also requires Swift SDKs.)

Related links:

• A Vision for WebAssembly Support in Swift

[kkebo]

• Add jobs for Linux static SDK and Wasm SDK builds #142

[kkebo]

The PR #142 has resolved this issue for swift build, however, it still can't run unit tests.