Add experimental wasm `externref` lang type #217950

Job	Run time
Calculate job matrix	32s
PR - x86_64-gnu-miri	1h 4m 44s
PR - pr-check-1	58m 32s
PR - tidy	5m 42s
PR - x86_64-gnu-llvm-21	1h 14m 15s
PR - pr-check-2	53m 2s
PR - x86_64-gnu-tools	1h 2m 47s
PR - aarch64-gnu-llvm-21-1	1h 4m 25s
PR - x86_64-gnu-gcc	1h 14m 14s
PR - aarch64-gnu-llvm-21-2	47m 17s
PR - x86_64-gnu-next-trait-solver-polonius	1h 4m 15s
PR - x86_64-gnu-gcc-core-tests	15m 27s
publish toolstate	1s
	9h 45m 13s

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