feat: add Blockly.Extensions.isRegistered function

[aaimio]

The basics

• I branched from develop
• My pull request is against develop
• My code follows the style guide

The details

Resolves

Proposed Changes

Adds a Blockly.Extensions.isRegistered function.

I'm using the block-plus-mins plugin which attempts to Blockly.Extensions.unregister an extension called controls_if_mutator. That extension only exists if you import everything from Blockly (default blocks etc.). However, In my project I'm importing Blockly as:

import * as Blockly from "blockly/core";

which means the controls_if_mutator extension is not there.

Currently, any call to Blockly.Extensions.unregister will put a warning in the developer console (if the extension doesn't exist).

Behavior Before Change

If attempting to unregister an extension that isn't registered, it will emit a warning to the developer's console.

Behavior After Change

This change will allow us to update the block-plus-mins plugin to check whether an extension is registered before unregistering it, so we can prevent the console.warn from happening.

Reason for Changes

Please see Proposed changes.

Test Coverage

N/A

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[aaimio]

@googlebot I signed it!

[moniika]

Thanks for the PR!

This change looks good to me, it just needs to be updated with the latest changes from develop. This quarter we did a large refactor to migrate to closure modules and the syntax and style of declaring functions has changed slightly.

If it easier to create a new PR, I can also review that instead.

[moniika]

I've manually resolved the merge conflict, no action needed.
Thanks again for the PR!

[aaimio]

Thank you @moniika!