Change string escaping to match Excel's behavior

[HactarCE]

Right now, Quadratic formulas use \ as the escape character. Excel doesn't do this; instead, " can be escaped using "".

And similarly in single-quoted strings: 'don''t' is equivalent to "don't"

In addition to updating parsing, be sure to update:

• [ ] escape_string() (in quadratic_core::formulas as of writing)

[luke-quadratic]

Advanced spreadsheet usage that may be irrelevant for our audience atm, plz reopen if desired as a feature/higher prio

[HactarCE]

Reopening, since this is a compatibility concern. We should either change all formulas on import from Excel, or we could support Excel's string escaping.