Investigate how to make .pyi stubs work in --editable installs

[feltech]

What

Investigate whether and how IDEs can make use of .pyi stub files for our _openassetio module in a pip --editable install.

Why

We do not install .pyi stub files as part of a pip --editable install, since during development of #1306 neither PyCharm nor VS Code could made to make use of them. The effort to make this work was abandoned due to running out the clock, and this follow-up issue made instead.

During development of OpenAssetIO itself it would be very useful to work in --editable mode, such that IDEs provide code completions for both a recently compiled extension module as well as the live in-progress sources. Currently we have to choose one or the other.