docs: Update context menu codelab for v12 (re-try)

[rpbourret]

The basics

• I validated my changes

The details

Resolves

v12 made two major changes to context menus:

• Added a focusedNode property to the scope argument
• Added support for separators

The codelab also was inconsistent in its use of the terms item/registry item (the thing in the registry) and menu option (the thing in the context menu).

Proposed Changes

In the codelab doc:

• Remove scopeType and replace uses of scopeType with scope.focusedNode
• Remove the section Scope/Block scope; restricting menu options to blocks is now illustrated in the Precondition: Node type section.
• Add a section on Separators
• Clarify/clean up uses of the terms item and option.
• Minor formatting cleanup. (Copied code is now indented properly for pasting.)

In the codelab code:

• Update starter-code/index.js and complete-code/index.js accordingly.

Reason for Changes

Current codelab is not up to date.

Test Coverage

Tested by hand.

Documentation

PR includes documentation fixes.

Additional Information

[maribethb]

Mostly some suggestions you should feel free to push back on, but you will have to decide one way or the other about the workspace comment bit.

[maribethb]

LGTM with minor wording quibble

[maribethb]

Thus, testing for what something isn't

How about... Thus, testing for specific types rather than allowing all (or all but certain types) will ensure that context menus are not shown on more components than you anticipated.

I'm not sure if that's actually better. If you don't like that, maybe putting isn't in italics would help make it read the appropriate emphasis?