Add to linter

[evykassirer]

two things I think would be good to add to our linter

you can reference https://github.com/socraticorg/mathsteps/issues/44 for instructions and examples from previous additions to the linter, or feel free to comment below and I can help you look into it

• [ ] use single quotation marks always, not double
• [ ] always have exactly one new line at the end of a file

[evykassirer]

thanks @fdagostino !