Rewrite of pyairtable.formulas

[mesozoic]

This is a breaking change to the pyairtable.formulas module, so it would have to be released as 3.0, but it introduces a more thorough way for library users to construct formulas and chain them together.

This module allows for complex nested expressions that are built up through chaining; for example:

>>> formula = match({"Customer": "Alice"})
>>> formula
EQ(Field('Customer'), 'Alice')
>>> complex = formula & Field('Delivery Date').gte(TODAY())
>>> complex
AND(EQ(Field('Customer'), 'Alice'), GTE(Field('Delivery Date'), TODAY()))
>>> str(complex)
"AND({Customer}='Alice', {Delivery Date}>=TODAY())"

The way all of the functions are exposed is a bit janky, and relies on code generation running against a text file that must be refreshed whenever Airtable releases new functions. I'll look to incorporate the approach laid out in #328 for reading those from the formula playground instead.

A summary of changes:

• formula= kwargs now accept either str or an instance of Formula
• The new Formula class has subclasses Comparison and FunctionCall
• The match() function signature remains the same, though it returns a Formula class instead of str
• to_airtable_value() is removed, and replaced with a similar (but not same) to_formula_str()
• field_name() replaces the FIELD() function
• EQ, NE, GT, GTE, LT, LTE replace equivalent functions (EQUAL(), etc.)
• Functions like AND() and OR() now return Comparison (a subclass of Formula) instead of str
• All Airtable formula functions are exposed, and return FunctionCall (a subclass of Formula)
• ORM fields descriptors contain .eq(), etc. convenience methods for producing formulas.

Resolves #304 Resolves #305 Resolves #324 Resolves #328

[codecov[bot]]

Codecov Report

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

Project coverage is 99.95%. Comparing base (b676bf0) to head (be844ec).

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #329 +/- ## ========================================== + Coverage 99.60% 99.95% +0.35% ========================================== Files 25 25 Lines 2025 2307 +282 ========================================== + Hits 2017 2306 +289 + Misses 8 1 -7 ```

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