fix: Let old block factory overwrite user defined blocks.

[johnnesky]

The basics

• I validated my changes

The details

Resolves

Fixes #6820

Proposed Changes

Makes a record of all of the reserved block type names that are required by the old block factory workspace on startup, and uses that to determine whether a user-defined block type has a conflicting name, instead of assuming the current definitions in Blockly.Blocks is an authoritative proxy for the reserved types.

Reason for Changes

The desired behavior is to prevent replacing critical blocks that are used by the block factory, like core Blockly blocks or even block-factory-specific blocks like factory_base. The existing strategy is to refuse to overwrite any block type that is currently recognized by the definitions in Blockly.Blocks, and to support this strategy, whenever the factory defines a user-defined block in this mapping, it always removes the block immediately after adding it so that user-defined block types become available again while critical block types remain reserved. However, there are ways that user-defined block types can "leak" into Blockly.Blocks permanently, after which point the block factory will mistake it for a critical block that shouldn't be replaced. For example, if there are already user-defined blocks saved in the block library when the block factory is reloaded or you switch between the factory/exporter tabs, all user-defined blocks will be added to Blockly.Blocks without being removed, so you're not allowed to make edits to saved blocks after reloading or switching tabs.

Test Coverage

There are no tests for the old block factory. It seems to work in my manual testing.

Documentation

N/A

Additional Information

There are at least two publicly hosted copies of the old block factory that ought to be updated:
https://blockly-demo.appspot.com/static/demos/blockfactory/index.html
https://google.github.io/blockly/demos/blockfactory/index.html