Registers a field type. Blockly.fieldRegistry.fromJson uses this registry to find the appropriate field type.
The field type name as used in the JSON definition.
The field class containing a fromJson function that can construct an instance of the field.
if the type name is empty, the field is already registered, or the fieldClass is not an object containing a fromJson function.
Registers a field type. Blockly.fieldRegistry.fromJson uses this registry to find the appropriate field type.