Autocomplete Slice #2: add warning annotation if a variable expression is known not to be defined

[twschiller]

[This issue was automatically opened using ProductBoard.]

Specification:

https://www.notion.so/pixiebrix/Autocomplete-Autosuggest-Expressions-270c903d6bf043848fd1a3dcd4cffe45

• Slice #2: add warning annotation if a variable expression is known not to be defined

  • Determine if the expression does not appear in the “may” set for that variable

  • If a variable expression, warn if it’s missing a @ prefix

[twschiller]

Link to subfeature: https://pixiebrix.productboard.com/feature-board/planning/features/15787559