Add formulas for missing logical operators

[MichalKacprzak99]

Implements missing formulas from section "Logical operators and functions" from documentation of "Formula field reference". Currently only EQUAL is implemented.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (e62a24e) 99.60% compared to head (65d579c) 99.61%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #323 +/- ## ======================================= Coverage 99.60% 99.61% ======================================= Files 24 24 Lines 1791 1801 +10 ======================================= + Hits 1784 1794 +10 Misses 7 7 ```

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[mesozoic]

I know this follows the existing pattern, but I'll admit that I'm not sure the current layout of the formulas module adds a lot of value. It doesn't strike me as being any easier to type formulas.GREATER_THAN(a, b) than the equivalent f"{a} > {b}".

I'm working on a branch that will (among other things) apply to_airtable_value to all function parameters, so if they're decimals or dates or some other type that needs Airtable-specific formatting, it's not the responsibility of the caller. But that branch is going to be a breaking change for a 3.0 release, because there's no real way to do it in a totally backwards-compatible manner.

I'm going to merge this branch because tests pass and it follows existing convention, but I will probably still propose breaking changes to this module for the next major release.

[mesozoic]

Thanks for contributing!