Add Swift 6 compatibility for async function code generation

[Choochmeque]

Add Swift 6 strict concurrency compatibility for generated async function code
Add @MainActor to test classes using XCTContext.runActivity
Add CI matrix to test with both Swift 5.0 and Swift 6.0

[Choochmeque]

Hi @chinedufn,
generated async swift code was not compiling with swift 6. This patch should fix it.
Could you please review it when you have time.
Also, could you please make a new release after merge this PR?

[Choochmeque]

Hi @chinedufn,
any updates on this PR?

[chinedufn]

Looks good thanks. Just one minor piece of feedback about making it clear that if Swift 5 ever becomes too difficult to maintain we can remove it from the test matrix.

[chinedufn]

FYI, our minimum support Swift version is 6.0, so if 5.0 ever causes problems we can remove it

swift-bridge/README.md

Lines 170 to 173 in 82b0885

	## Minimum Supported Swift Version (MSSV)
	`swift-bridge` currently guarantees that the Swift code that it generates will work on Swift `6.0` and later.
	This is known the project's "Minimum Supported Swift Version" (MSSV).

Let's leave leave a comment here in this test.yml. Something like:

# NOTE: As explained in `README.md`, the minimum supported Swift version is currently `6.0`.
#   If supporting `5.0` ever becomes difficult we can consider removing it from the test matrix.

[chinedufn]

Thanks for the clear documentation about safety.

[chinedufn]

Published as 0.1.59 https://crates.io/crates/swift-bridge/0.1.59