Add support for WASM backend #3339

Job	Run time
unit-tests	13s
cabal	1m 11s
nix-build	11m 12s
runghc	1m 10s
multi-target	1m 43s
hix	1h 4m 3s
tests-benchmarks	20s
maintainer-scripts	18s
build-extra-hackages	16s
shellFor-single-package	3m 7s
closure-size	8s
docs	2h 16m 35s
hydra-ifdLevel-3	30m 18s
shellFor-hoogle	20s
update-docs	10s
check-materialization-concurrency	26s
haskell-nix-roots-do-not-require-IFDs	33s
hydra-ifdLevel-2	15m 51s
cabal-doExactConfig	17s
shellFor-not-depends	30s
shellFor-multiple-package	1m 11s
hydra-ifdLevel-0-and-1	7m 26s
check-path-support	26s
plan-extra-hackages	16s
check-nix-tools	23s
hix-cabal	20s
hydra-without-remote-builders-ghc967	2m 0s
hydra-without-remote-builders-ghc9102	3m 30s
	4h 44m 13s

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