Feature request: double and triple clicking should select in MathQuills

[jwmerrill]

In (most?) browsers, double clicking a word selects it, and triple clicking selects an entire paragraph.

In MathQuill, double and triple clicking currently do nothing, as far as I can tell.

Proposal:

• Double clicking should select the least "math block" ancestor. This would be roughly similar to e.g. Shift-End from the keyboard, but would select an entire block instead of just the part of it that is to the right of the cursor

  mathquill/src/services/keystroke.ts

  Lines 56 to 58 in aab6ed2

  	case 'Shift-End':
  	ctrlr.selectToBlockEndInDir(R);
  	break;

• Triple clicking should select all in the current mathquill

This feature request is somewhat important to Desmos because Desmos had previously disabled mouse events on static mathquills, which allowed the user to get native selection behavior on static mathquills and in particular, allowed them to double or triple click on static mathquills used to display calculator expression evaluations.

Desmos is now using MQ's mouse events on static mathquills, which is an improvement in some ways, but turns out to be a regression in terms of double and triple click behavior.