repl: Catch syntax errors better
Replace the growing list of 'isSyntaxError' whackamole conditions with a
smarter approach. This creates a vm Script object *first*, which will
parse the code and raise a SyntaxError right away.
We still do need the test function, but only because strict mode syntax
errors are not recoverable, and should be raised right away. Really, we
should probably *only* continue on "unexpected end of input" SyntaxErrors.
Also fixes a very difficult-to-test nit where the '...' indentation is
not properly cleared when you ^C out of a syntax error.
Closes #6093