[sjakobi]

Closes #784.

Sample output:

$ ./build --list-targets
all - Build everything
clean - Remove _build
test - Run the test suite
_build/app
_build/docs/*.html
$ ./build --list-targets='_build/**'
_build/app
_build/docs/*.html

I went with the more explicit --list-targets name instead of the originally proposed --targets. If you'd prefer something else (@bgamari proposed --help-targets in https://gitlab.haskell.org/ghc/ghc/-/work_items/18015), I'd be happy to switch to a different name.

[sjakobi]

@ndmitchell Would you accept a follow-up change to remove the target list from --help output? IMHO the status quo is pretty noisy and distracts from the flag info.