[Enhancement] Add Support for Arithmetic Operations

**Is your feature request related to a problem? Please describe.**

I'd like to do some basic math in assertions and counts() outside of the binary comparison operations.

**Describe the solution you'd like**

Be able to parse and validate a rule similar to the following:
```
let minimum_tag_ratio = 0.95
rule tag_ratio_must_be_over_sla {
let all_resources_to_check = Resources.*[ Type == /AWS::IAM::Role/ ]
let found_required_tags = Resources.*[
Type == /AWS::IAM::Role/
Properties.Tags[*].Key == ...blahblah
...blah
]
(count(%found_required_tags) / count(%all_resources_to_check)) >= %minimum_tag_ratio
}
```

The issue names no files, tests, or entry points. Start by locating the parser and rule-evaluation paths for expressions such as count() and binary comparisons, then trace how the example ratio rule is parsed and validated. Done means arithmetic expressions, including division and comparison with a variable, are supported with test coverage for the example.

Written by the indexing model from the issue text.