add @available(*, noasync) attribute

[lukaskollmer]

add @available(*, noasync) attribute

♻️ Current situation & Problem

changes the @ThreadLocal macro to annotate the get/set accessors it defines as @available(*, noasync).
the reason for this change is that the ThreadLocal mechanism completely does not work in async code, since any suspension point might result in the function switching to a different thread.
the reason why we weren't already doing this, is that i wasn't aware of the existence of this attribute.

⚙️ Release Notes

• property accessors created via the @ThreadLocal macro now are marked as @available(*, noasync).

📚 Documentation

the fact that the mechanism shouldn't be used in async code already was explicitly documented

✅ Testing

no new tests; existing tests are adjusted
(i tried writing a test to check that a program that accesses an @ThreadLocal variable from an async function fails to compile, but as it turns out you can only test the output a macro will produce, but not for any downstream compilation errors...)

Code of Conduct & Contributing Guidelines

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• I agree to follow the Code of Conduct and [Contributing Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 71.88%. Comparing base (2600d76) to head (9e296f4).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main       #4   +/-   ##
=======================================
  Coverage   71.88%   71.88%           
=======================================
  Files           4        4           
  Lines          32       32           
=======================================
  Hits           23       23           
  Misses          9        9           

Files with missing lines	Coverage Δ
Sources/ThreadLocal/ThreadLocal.swift	73.69% <ø> (ø)
Sources/ThreadLocalMacros/ThreadLocalMacro.swift	87.50% <ø> (ø)

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