Convert generateFieldTestBlocks in dev-tools to use JSON

[rachel-fenichel]

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Component

dev-tools

Problem

I want to automatically generate lots of test blocks, as generateFieldTestBlocks does, but I also want to add other blocks to the generated toolbox definition.

Request

• Write generateFieldTestBlocksJson to generate JSON toolbox definitions instead of XML toolbox definitions.
• Change generateFieldTestBlocks to wrap the JSON version and convert it to XML.

Alternatives considered

No response

Additional context

Encountered while working on #2162