Implement WGSL compound assignment statement, `+=`, `-=`, et al.
<rdar://problem/108347694>
Pull request: https://github.com/WebKit/WebKit/pull/13001
Committed 263272@main (56d070db0af6): <https://commits.webkit.org/263272@main> Reviewed commits have been landed. Closing PR #13001 and removing active labels.