Add warning when `arg` is used in subtraction.

[sovdeeth]

Suggestion

When the arg expression is used in subtraction, like arg - 1, add a warning about this being interpreted as subtraction, not as arg 1. Suggest using arg or arg 1 instead.

Why?

Many users get confused when arg-1 / 10 is parsed as arg - (1/10) instead of arg / 10 when using commands with 1 argument. A warning would help clarify these issues.

Other

See #7733

Agreement

• [x] I have read the guidelines above and affirm I am following them with this suggestion.