Delete fields that were moved to samples out of core

[BeksOmega]

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Problem

To actually make Blockly smaller when compressed, we need to delete the fields that were moved into samples out of core.

Request

Delete the fields from core.

Alternatives considered

N/A

Additional context

Dependent on #7225

Issues that tracked moving the fields:
RaspberryPiFoundation/blockly-samples#1646
RaspberryPiFoundation/blockly-samples#1645
RaspberryPiFoundation/blockly-samples#1644

[maribethb]

We put this in the Q3 project, but we haven't decided if we're doing a major release yet or not this quarter. So a PR for this should not be submitted until we've confirmed we plan to do a major release as this is a breaking change.

[maribethb]

Note RaspberryPiFoundation/blockly-samples#1733 needs to be fixed before this can be submitted as well.

[rachel-fenichel]

Issues to track additional work for moving block definitions into the field plugins:

• Field colour: add block definitions blockly-samples#2152
• Field angle: add block definitions blockly-samples#2153
• Multiline text field: add block definitions blockly-samples#2154
• Verify code demo will not be impacted by moving blocks to plugins #7826

[rachel-fenichel]

We also need to validate that all of the register functions (registerMutator, registerMixin, etc) can handle registering the same extension multiple times. The desired behaviour is:

• Check for any other functions that need to have the same behaviour
• Check the currently registered function and ensure it matches the one attempting to be registered. If so, silently accept the registration call and do nothing. If they don’t match, emit a warning or error.
• Update if needed

Rationale: An accepted pattern of use for fields is to unregister a field class, then register a substitute class under the same name. This allows users to install a default Blockly block but change the underlying field to one with different functionality.

Documentation: We may need to instruct users to unregister fields only after installing any blocks, so their objects are not overwritten by the built-in objects.