Add editable install script

[sbidoul]

This is a much simpler and robust installation method.

~The drawback is that it is a little bit slow when using build isolation on large repos, because pip does not cache build environments yet.~

~So here we use --no-build-isolation, but there is a small chance that some third party dependency would not work in this mode if it requires a build system that is not installed.~

To address the performance issue, we use uv.

This mode is off by default, and is enabled with an undocumented and temporary environment variable: OCA_INSTALL_ADDONS_MODE=editable.