Move the package version to VERSION.txt.

[grynspan]

This change moves the definition of Swift Testing's version to a new file, VERSION.txt. This file doesn't represent a standard, per se, but it is common for open-source projects to list their version info in such a file. And by putting our version info in this file, we can share it between package builds and CMake builds.

Currently using C23's #embed which is supported in clang as a non-standard extension. C++26 will add support for #embed.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[grynspan]

@swift-ci test

[stmontgomery]

Also, looks like there's a conflict with my PR #1312 since I touched many CMake files

[grynspan]

Will fix the conflict.

[3405691582]

FYI: for OpenBSD, the native platform compiler doesn't support C23 yet, so this causes an error (function-like macro '__has_embed' is not defined). I'll send a pr soon to add an extra __STDC_VERSION__ check around this after some testing.

[grynspan]

We should always be building with (a recent) clang, but if there's an OpenBSD-specific issue, can it be solved by setting the SWT constant at build time?

[3405691582]

I don't think so, but instead, we could probably also do something like a platform specific -D__has_embed(x)=0 to avoid the issue too, which might be a little neater.

[grynspan]

Or just #if defined(__has_embed) && __has_embed(...) ... no?

[3405691582]

That won't work on older clang versions (see e.g. https://godbolt.org/z/edGb78fbd).