fix($parse): add `.assign()` function to settable expressions in CSP mode

Due to changes to CSP-mode detection and the CSP api in general, the test suite was not able to
report regressions related to CSP.

This CL corrects the broken test suite and broken CSP detection mechanics in $parse, and corrects
the regression introduced where assignable expressions would not have an `assign` routine in CSP
mode.

Closes #9048