Max variable linter: not taking into account recently increased limit

[jeskew]

The max-variables linter issues an Error-level diagnostic when a template has more than 256 variables. According to the docs page on ARM template limits, the limit is now 512. (I confirmed in the deployment engine codebase that 512 is correct.) This could lead to false positives.

Moved to #15461:
The linter is also not counting variables imported directly or as part of the closure of another imported symbol. This could lead to false negatives.