fix: preserve toolbox category IDs

[gonfunko]

This PR copies the toolbox category ID to the correct key for modern Blockly. This is required for gonfunko/scratch-gui#8 to work end-to-end.

[BeksOmega]

Where is toolboxitemid referenced? I did a search in GUI and couldn't find it.

[gonfunko]

It's used when assigning the ID to the ToolboxItem in core: https://github.com/google/blockly/blob/185c82a991484e68b1a76825f2c4b082b0168677/core/toolbox/toolbox_item.ts#L46

Without this, core synthesizes an ID for the toolbox category, which doesn't match the ID Scratch knows about causing getToolboxItemById() to return null. Core apparently used to look for id a few years back but changed the key it uses.

[BeksOmega]

Well that's annoying. But I can see why they didn't want the id attribute in XML to conflict with DOM IDs :P

Do we have access to the actual defs? and if so can we migrate them instead?

Assuming not, so approving based on that assumption.

[gonfunko]

Oh good call, yes. Reverting this PR in favor of gonfunko/scratch-vm#2
and gonfunko/scratch-gui#9.