Evaluate extending the Blockly Field.Dropdown for constants variable support

[zfi]

There are a large number of blocks that contain pin configuration settings, typically as a drop-down list of available I/O pins supported on the target device. These lists must support constant values in addition to the pins enumerated in the block.

Rather than incorporate logic in each block to manage any constant values that might appear in the drop-down list, it is probably a better approach to extend the base Blockly.Field.Dropdown class with the logic necessary to manage constants within the list.