Custom fields in extensions

[picklesrus]

Got a report that Blockly.Field.register isn't working for field types that aren't yet defined. Seems to work okay to override existing types, but not new ones.

Needs more investigation.

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Moved from scratchfoundation/scratch-blocks#1725
/cc @picklesrus

[picklesrus]

The problem is really when trying to use a custom field in an extension. You can register it fine using the blocks register, but the VM doesn't know how to map the field to the correct thing.