Pass the kind of build (regular/editable) to meson

This is useful for pandas, where we would not want to install conftest.py (would break pytest due to multiple conftest.py!) on an editable install, but would want to do it on a release build(we have a function ``pandas.test()`` that allows users to test their pandas in the public API).

Start by tracing the Python build backend's regular and editable build entry points and how each invokes Meson. The change is complete when Meson receives whether the build is regular or editable, allowing projects such as pandas to install conftest.py only for release builds.

Written by the indexing model from the issue text.