Add "showdefinition" to Vim syntax highlight #992

Job	Run time
build-and-test (macos-13, 15.2, true, vimtags, check-xcodeproj-compat) / build-and-test	17m 1s
build-and-test (macos-14, 15.4, false) / build-and-test	9m 25s
build-and-test (macos-15, 16.4, true, true, true) / build-and-test	26m 29s
build-and-test (macos-13-xcode14-self-hosted, true, _10.9, true, true) / build-and-test	0s
	52m 55s

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