bpo-40059: Add tomllib (PEP-680) (GH-31498)

bpo-40059: Add tomllib (PEP-680) (GH-31498)

This adds a new standard library module, `tomllib`, for parsing TOML.

The implementation is based on Tomli (https://github.com/hukkin/tomli).

## Steps taken (converting `tomli` to `tomllib`)

- Move everything in `tomli:src/tomli` to `Lib/tomllib`. Exclude `py.typed`.
- Remove `__version__ = ...` line from `Lib/tomllib/__init__.py`
- Move everything in `tomli:tests` to `Lib/test/test_tomllib`. Exclude the following test data dirs recursively:
  - `tomli:tests/data/invalid/_external/`
  - `tomli:tests/data/valid/_external/`
- Create `Lib/test/test_tomllib/__main__.py`:

  ```python
  import unittest

  from . import load_tests

  unittest.main()
  ```

- Add the following to `Lib/test/test_tomllib/__init__.py`:

  ```python
  import os
  from test.support import load_package_tests

  def load_tests(*args):
      return load_package_tests(os.path.dirname(__file__), *args)
  ```

  Also change `import tomli as tomllib` to `import tomllib`.

- In `cpython/Lib/tomllib/_parser.py` replace `__fp` with `fp` and `__s` with
  `s`. Add the `/` to `load` and `loads` function signatures.

- Run `make regen-stdlib-module-names`

- Create `Doc/library/tomllib.rst` and reference it in `Doc/library/fileformats.rst`