[WIP] Add minimal CMake support

[kabiroberai]

Adds enough CMake support to get WasmKitWASI importing in the Swift compiler's build system. Marking as a draft until I can fully test this.

[kabiroberai]

Was able to confirm that this gets things building correctly. I based the Swift CMake utils off of the ones in swift-syntax; I don't have too much experience with CMake though so let me know if there's any glaring issues or just things I may not have considered in general cc @kateinoigakukun

[kateinoigakukun]

LGTM! I'll add CI checks

[kateinoigakukun]

We might be able to use CMake's native Swift support, but let's go ahead for now.

[kabiroberai]

We might be able to use CMake's native Swift support

This is indeed what this PR uses fyi — the AddSwiftHostLibrary utils layer on top of CMake's built-in Swift support. I'm not sure why there's explicit handling for swiftmodules though, so maybe it's worth looking into whether that's redundant but it gets the job done right now so I don't want to cause any regressions by removing it.