Encode Formulas to prevent CSV injection

[maennchen]

If this is a feature request, why do we need it?

• We are using this library to generate CSVs for user-generated input and transmitting them to another party. (Which sadly requires a CSV and not a sane format like a JSON)
• We did a pentest on the application
  • Security Advisory: GHSA-8pwv-jhj2-2369
• The pentest uncovered a CSV Injection vulnerability, if the user-generated input includes formulas like =1+1

To address this, I suggest to implement encoding of =1+1 as "'=1+1"

If this is a bug, steps to reproduce it

• Generate a CSV with a cell value of =1+1
• Open with Excel
• See value 2
• Is your input CSV RFC 4180 compliant? - Yes
• Are you interested in helping with a PR? - Yes

[beatrichartz]

PR is merged, thanks