refactor!: Update and modernize the continuous toolbox plugin.

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• Fixes Update continuous toolbox plugin for flyout API changes #2464
• Fixes Importing continuous-toolbox has side effects #1041
• Fixes continuous-toolbox not created function  #857
• Fixes Continuous toolbox flyout auto close #1922

Proposed Changes

This PR updates the continuous toolbox plugin for compatibility with the revised flyout APIs introduced in Blockly v12. It also converts the plugin to Typescript.

Additionally, it fixes several existing issues:

• Importing the plugin is free of side effects; the plugin must be registered and injected to take effect. Previously, the CSS and some classes were unconditionally registered.
• The continuous flyout now supports autoclosing; users can simply call Blockly.getMainWorkspace().getFlyout().setAutoClose(true) to enable this behavior.
• Mutations to procedure blocks in the main workspace will be immediately reflected in the flyout.
• Extremely poor performance when adding/deleting large numbers of blocks to the workspace has been resolved by debouncing toolbox refreshes in response to block add/create/change events.
• The implementation has been slightly adjusted to make it more extensible in response to shortcomings identified by the Scratch modernization effort.

This PR should not be merged until Blockly v12 is released, as it depends upon APIs added in that version.

[cpcallen]

This PR should not be merged until Blockly v12 is released, as it depends upon APIs added in that version.

Might it be worth creating a v12 branch in this repo (if there is not already one) and retargeting this PR to it?

[gonfunko]

This PR should not be merged until Blockly v12 is released, as it depends upon APIs added in that version.

Might it be worth creating a v12 branch in this repo (if there is not already one) and retargeting this PR to it?

Done!

[gonfunko]

This also fixes #2471.

[gonfunko]

And also fixes google/blockly#8543