feat: add extension point for overriding Grid functionality

[riknoll]

The basics

• I validated my changes

The details

Resolves

Fixes #8614

Proposed Changes

Refactors the Grid class so that it can be overridden by a plugin. The static methods for parsing grid options and creating the SVG pattern element have also been moved onto a new GridProvider class which can be passed in as a plugin in the BlocklyOptions object. The implementations of these functions are unchanged; they just got moved around.

Originally, I just wanted these to be static methods on the IGrid interface, but the typings for the registry's getClass functions don't really allow for static properties and I didn't want to tear all that up in this PR.

I would love some feedback on the typing of GridOptions here. My example use case for this is the MakeCode Minecraft editor which overrides the grid with a background image that scrolls with the workspace. A plugin that provided that functionality would likely need to extend the GridOptions type with extra fields like the URL of the image, width/height, opacity, etc. However, the grid property of the BlocklyOptions object passed to inject is pretty strongly typed; the only way around it right now is to do a lot of casting to any.

Reason for Changes

See the linked issue for reasoning!

Test Coverage

Tested manually in the playground and by creating a test plugin locally. Happy to add unit tests, but I didn't see any for other extension points at a click glance.

Documentation

Additional Information

I can also add a plugin to blockly-samples that utilizes this change should this PR be approved.