Improve MacroTesting Documentation and Add Swift Testing Integration

[ajkolean]

This PR updates the documentation, comments, and examples in the Macro Testing library to reflect the use of the record parameter instead of the outdated isRecording bool.

Changes Include:

• Updated inline comments in AssertMacro.swift to clarify the record parameter's role and its integration with SNAPSHOT_TESTING_RECORD.
• Enhanced examples in the README to highlight how record can be used with withMacroTesting and the Swift Testing framework.
• Removed unnecessary @_spi(Experimental) annotations, aligning with conventions in the related SnapshotTesting library.
• Ensured that all references to isRecording are replaced with record for consistency.

What’s The Point:

• These changes ensure that users have accurate, clear, and consistent guidance when using the library.
• While the API itself remains unchanged, the improvements in documentation reduce confusion and make the library easier to adopt.

⠀### Backward Compatibility:

• No breaking changes were introduced, as this PR only updates documentation and comments.

[stephencelis]

This looks great, thanks for your help!