allow operators to procede dollar quoted strings

[proddata]

Thanks for contributing!

Before submitting your pull request please have a look at the following checklist:

• [x] ran the tests (pytest)
• [ ] all style issues addressed (flake8)
• [x] your changes are covered by tests
• [ ] your changes are documented, if needed

Fixes #763 Currently string literals using dollar quoting ($js$mytext$js$) must be preceded by a whitespace character, which prevents e.g. using SET application_name=$$foo;bar$$ which is valid PostgreSQL