## Summary of changes

### Refactor common functionality into base module
This allows support multiple JSON encoders by having common
functionality in `pythonjsonlogger.core` and then specialist formatters
for each encoder.

This is useful / needed, as not all JSON encoders support the
`json.dumps` or `json.JSONEncoder` interfaces exactly. This enables us
to support other JSON encoders like orjson and msgspec. In the future we
may add support for other encoders.

### Better handling for custom styles
Achieved by mimicking `logging.Formatter.__init__` without actually
calling it.

A code snippet is worth `2**10` words:

```python
from pythonjsonlogger.core import BaseJsonLogger

class CommaSupport(BaseJsonFormatter):
    def parse(self) -> list[str]:
        if isinstance(self._style, str) and self._style == ",":
            return self._fmt.split(",")
        return super().parse()

f = CommaSupport("message,asctime", style=",", validate=False)
```

### Rename `jsonlogger` module to `json` module

Compatibility is maintained for the moment using `__getattr__` in
`__init__`.

This is to enable more consistent naming of implementation specific
module names. It also stops throwing around the word "logger" when this
module only contains formatters.

### Add support for orjson
[orjson](https://github.com/ijl/orjson) is a high performance (and more
JSON spec correct) encoder. Given how many logging calls may occur -
having a performant formatter available is important.

This includes ensuring it is covered in tests on appropriate platforms.

Note: orjson is not supported on pypy, and currently does not build for
py313.

### Add support for msgspec
[msgspec](https://jcristharif.com/msgspec/index.html) is another library
containing a high performance JSON encoder.

Note: msgspec is not supported on pypy, and currently does not build for
py313.

### Drops python 3.7 support
This is primary due do making use of the
[`validate`](https://docs.python.org/3/library/logging.html#formatter-objects)
argument. I was also having issues with CI because python 3.7 is not support on most "latest"