Improved the handling of the ``config_settings`` parameter in both PEP 517 and
PEP 660 interfaces:

- It is possible now to pass both ``--global-option`` and ``--build-option``.
  As discussed in #1928, arbitrary arguments passed via ``--global-option``
  should be placed before the name of the setuptools' internal command, while
  ``--build-option`` should come after.

- Users can pass ``editable-mode=strict`` to select a strict behaviour for the
  editable installation.
