Make eraser snapping optional

In my opinion, snapping to assistants is pretty tiresome due to the fact that erasers also snap. This commit makes it so that eraser snapping is optional and defaults to off. This commit adds new strings.

Test Plan

Test if regular brushes properly snap regardless of the option. Test if erasers properly snap with Eraser Snapping is enabled. Test if erasers don't snap if Eraser Snapping is disabled.

Formalities Checklist

• I confirmed this builds.
• I confirmed Krita ran and the relevant functions work.
• I tested the relevant unit tests and can confirm they are not broken. (If not possible, don't hesitate to ask for help!)
• I made sure my commits build individually and have good descriptions as per KDE guidelines.
• I made sure my code conforms to the standards set in the HACKING file.
• I can confirm the code is licensed and attributed appropriately, and that unattributed code is mine, as per KDE Licensing Policy.