Add support for macos-15 for CI #782

Job	Run time
build-and-test (macos-13, 15.2, vimtags, check-xcodeproj-compat)	40m 57s
build-and-test (macos-14, 15.4, true)	35m 50s
build-and-test (macos-15, 16.2, true, true)	46m 20s
	2h 3m 7s

Provide feedback