feat: add block factory export in json

[maribethb]

The basics

• I validated my changes

The details

Resolves

Needed for google/blockly-samples#2291

Proposed Changes

Adds an "Export Block Library" button that downloads the block factory blocks in JSON format.

Reason for Changes

• This is needed to support uploading blocks to the new tool.
• Unfortunately I had to add a new button because using XML won't work. The block definitions used in this tool are not directly compatible with the ones in the new tool. Adjusting the JSON by hand to make them load-able in the new tool is 1000% easier than adjusting the XML by hand.
• I will add documentation explaining how to get blocks from one tool into the other.
• See also feat: support uploading file from old block factory blockly-samples#2336 for how this file is used

Test Coverage

Documentation

Coming soon!

Additional Information