[UI] Fix highlighting of return command and list accesses

[jpelay]

Description

• Fixes the incorrect name that was used to get the return value from the highlighting json
• Adds the Int and Number rule to ListAccess when appropriate. Functionally this does nothing since they were still being highlighted, but now the trees we generate don't have any error nodes.
• Allows use of list accesses on left hand side of assignments
• Fixes the highlighting of EqualityChecks using = on level 12 onward

Fixes #4857 and fixes #4856 and fixes #4854

How to test

Go to level 12 and write this program:

define sum with n1, n2
    return n1 + n2

Check that return is being highlighted as a keyword

Now go to level 16 and type:

print l at [1]

The closing bracket should be highlighted

Go to level 12 and type:

list at number = 1

On this same level type:

if name = 'Hedy the Robot'
    print 'Hi there!'

The = should be highlighted
The variables and the at should be highlighted

[Felienne]

Works!

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).

[mergify]

Thank you for contributing! Your pull request is now going on the merge train (choo choo! Do not click update from main anymore, and be sure to allow changes to be pushed to your fork).