Add missing `ToWgslString` impls for vecs #460

Job	Run time
run-examples	11m 47s
Check format	1m 17s
Coverage	3m 52s
build (2d, stable, windows-latest)	13m 21s
build (2d, stable, ubuntu-latest)	20m 53s
build (2d, stable, macos-latest)	4m 54s
build (3d, stable, windows-latest)	14m 8s
build (3d, stable, ubuntu-latest)	9m 25s
build (3d, stable, macos-latest)	7m 39s
build (all, stable, windows-latest)	10m 59s
build (all, stable, ubuntu-latest)	10m 32s
build (all, stable, macos-latest)	3m 12s
	1h 51m 59s

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