Should we make closure warnings into errors with `-Werror` and/or `-sSTRICT`

[sbc100]

Hopefully once this lands are libraries and all our test code should be free of closure warnings:
#16199.

Perhaps we should consider defaulting to -sCLOSURE_WARNINGS=error when -Werror and/or -sSTRICT (or both?) are used? Obviously users could opt out by setting -sCLOSURE_WARNINGS=.. themselves.