CLN: de-privatize _parsers and _engines in pandas/core/computation and rename to something more descriptive

As a followup from #28215 

_parsers and _engines as used in the code for the `.query` and `.eval` methods should be de-privatized and named to something more informative. 

```
_engines: Dict[str, Type[AbstractEngine]] = {
    "numexpr": NumExprEngine,
    "python": PythonEngine,
}
```

```
_parsers = {"python": PythonExprVisitor, "pandas": PandasExprVisitor}
```

@jreback 