Improve error message for an invalid rule has syntax

[jmsfltchr]

What is the goal of this PR?

Users could quite sensibly think that they can assign a type to a variable attribute in a rule then. This is not permitted because the type of a variable in the rule then has already been determined by the rule when. Users should be clearly told what the exact issue is and how to remedy this.

What are the changes implemented in this PR?

Tell the use the exact type and variable causing the issue and suggest to them to remove the type.
For example:

rule people-are-the-same:
when {
      $p isa person, has name $n;
    } then {
      $p has name $n;
    };

throws error:

[TQL31] TypeQL Error: Rule 'people-are-the-same' 'then' '$p has name $n' tries to assign type 'name' to variable 'n', but this variable already had a type assigned by the rule 'when'. Try omitting this type assignment.

[jmsfltchr]

Ideally variable 'n' should be variable '$n' - can we do this?

[flyingsilverfin]

why the python-style variable naming??

[jmsfltchr]

Only because I wrote some Python recently 🤣 fixed now

[flyingsilverfin]

don't forget to change the status to "in review" when it's ready to review, and also assign me as reviewer: both are required for it to turn up on my github board! I do tend to use this a lot when im only online sporadically as a quick summary of what's on my plate :)