add 'field helper' concept to connections

[Hamms]

The concept here is that we want to be able to add the ability for certain inputs/connections to augment the fields being attached to them with certain helpers. Specifically, we want for Artist blocks with value inputs like this one

To still be able to show the Angle Helper:

This PR proposes a generic system to do that. When we define the inputs for the block, in addition to validation checks we can add a Field Helper like so:

  blockly.Blocks.draw_turn = {
    // Block for turning left or right.
    helpUrl: '',
    init: function () {
      this.setHSV(184, 1.00, 0.74);
      this.appendValueInput('VALUE')
        .setCheck(blockly.BlockValueType.NUMBER)
        .addFieldHelper(blockly.BlockFieldHelper.ANGLE_HELPER, {
          block: this,
          directionTitle: 'DIR',
        })
  ...

And then Fields can consult getFieldHelperOptions_ from within showEditor_ to see if they are (at the time of editing) in a connection hierarchy that wants them to show a helper, meaning we can dynamically display the angle helper based not on the field itself, but based on the input it's connected to!

I'd love to get y'all's general thoughts on this approach.

[balderdash]

This looks great!

[ryansloan]

🎉