Export registry objects, not classes, from contextmenu registry and shortcut registry

[rachel-fenichel]

Is your feature request related to a problem? Please describe.

Both the context menu registry and the shortcut registry export a class which has a static property, registry, which is the singleton that should be interacted with. This is confusing and makes it look like calling the constructor is a reasonable thing to do.

Describe the solution you'd like

Add exports:
exports.shortcutRegistry = ShortcutRegistry.registry and exports.contextMenuRegistry = ContextMenuRegistry.registry. Notice the lowercase first letters, to distinguish from exporting the class.

Remove the class exports.

Update related code and documentation.

Describe alternatives you've considered

Change the registries to be associated with workspaces, rather than a global singleton.

Additional context

I started to do the suggested change in #5940 (comment) but Maribeth pointed out that it was an API change in the middle of a refactor, so I backed it out.

[cpcallen]

As I have mentioned previously, I think it would be great if we were to eventually move to fully-independent Blockly instances (i.e., with no global state), so I'd probably opt to continue exporting the class constructor but add the instance as a second named export (for the time being).

But of course we can certainly revisit down the road, so no reason to keep it if you think it is confusing.