Add compatibility for Embedded Swift

[MaxDesiatov]

Summary

When building swift-markdown with Embedded Swift for Wasm, multiple issues occur:

1. Dependency on Foundation doesn't allow building for Embedded Swift, but this dependency is not required for most Embedded Swift use cases
2. Recursive generics with ManagedBuffer case compiler hangs, which should emit error diagnostics instead, fixed in Embedded: fix OOM in swift-frontend with recursive generics swift#88296

Dependencies

swiftlang/swift#88296 is not a strict dependency, since it will only cause the build of main to fail instead of hang. We're working around the use of recursive ManagedBuffer when building for Embedded Swift altogether in this PR.

Testing

Existing non-embedded builds test suite passes. Since Swift Testing doesn't support Embedded Swift yet, this configuration is verified manually.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test

[MaxDesiatov]

@swift-ci test