docs: Update context menu codelab for v12

[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 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

[rpbourret]

Closed due to incorrect base.