This commit refactors the shim host to be agnostic to the shims being
generated, and provides an API for generating additional shims besides
the .ngfactory.js. This will be used in a following commit to generate
.ngsummary.js shims.
Testing strategy: this refactor introduces no new behavior, so it's
sufficient that the existing tests for factory shim generation continue
to pass.
PR Close#26495
This simple refactor of the build rules renames the .ngfactory.js shim
generator to 'shims' instead of 'factories', in preparation for adding
.ngsummary.js shim generation.
Testing strategy: this commit does not introduce any new behavior and
merely moves files and symbols around. It's sufficient that the existing
ngtsc tests pass.
PR Close#26495
