This should be well covered in the syntax templates and opens the path for tests with module code restrictions

- Add cases for mixing module and script code
- Rename test case from return promise to thenable
- Fix script code case with valid loaded fixture
- Add a test to assert a promise return
- Add case for specifier toString rejection
- Add case for specifier toString
- Test Assignment expression abrupt completion
- Test Promise return

- Add cases for invalid syntax
- Add valid cases
- nested imports
- add non existent file case
- Fix cases and templates to use a full importcall expr token
- add case for call expression position
- remove unnecessary module flag from templates
- Add templates for nested with

Fixes #1829