Delete sqrt with 'delete' before it, and add 'delete' tests

[jared-hughes]

Behavior change

Setup: cursor immediately before a square root, e.g. at the start of a field with latex \sqrt{x}

Before: pressing "delete" (delete-to-the-right) would move the cursor to the right.

After: pressing "delete" (delete-to-the-right) deletes the square root and keeps the content within.

This matches the behavior of pressing "delete" when the cursor is at the far-right of the radix/index/degree of an nthroot.

Implementation notes

It just does the same as typing "Right" then "Backspace" (delete-to-the-left).

I also put some tests for "Delete" since there were previously only tests for "Backspace," added in mathquill@4a8779f.

[mikehaverstock]

Code looks good. Behavior seems to be as described. I think someone else should confirm this is the most intuitive/consistent behavior.

[jared-hughes]

Merge blocker (now fixed): Negative behavior change when pressing "delete" before the index of an nthroot: <cursor>\sqrt[3]{5}. PR leaves "35" behind. Old behavior is moving the cursor to the start of the "3".