Implement "strict mode" where referencing an undefined variable in ${nonExistingVar} would fail

### Use case

Currently, a typo in variable name is hard to spot as it might silently pass to the downstream system unnoticed.

### Possible solution

It might be helpful if we implement opt-in "strict mode" so referencing unknown variables becomes error rather than acceptable behavior.

See:
* https://perldoc.perl.org/strict
* https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Strict_mode

### Possible workarounds

_No response_

### JMeter Version

5.6

### Java Version

_No response_

### OS Version

_No response_