feat: remove unnecessary escape sequences in strings

[shannonrothe]

This fix ensures any unnecessary escape sequences in a string literal are omitted. It is largely based off the rules set out here. I've modified remove_needless_quote_backslashes to incorporate the additional "rules", whereby previously it only cared about escaped quotes depending on the type of string literal.

I'm pretty heavily relying on a few new test cases + ensuring existing tests didn't break, so there's a chance I might miss some edge cases

[CLAassistant]

All committers have signed the CLA.

[dsherret]

Thanks! This seems very nice to have. However, when testing out what prettier does, it maintains escapes for numbers since "\5" !== "5". If we merge this I want to be absolute sure we can actually do this. It might be good to write a script in javascript that goes through all the character possibilities and finds all the scenarios where a character is not equal to itself escaped.

[dsherret]

I was just thinking more about this and actually I think we should change this to only occur when the char is also .is_ascii_alphabetic(). That will be much safer to do.