Rename FieldAngle to ScratchAnglePicker

[rachel-fenichel]

A lot of the angle field code is copied over from blockly, and it's registered under the same name. There's no requirement to keep the same class name since the registration name is all that matters. Current naming sometimes makes it difficult to tell if a change is in a blockly field or a scratch field.

This is a suggested cleanup but not a requirement. Other fields may also benefit from a similar analysis.