feat: add rattler_menuinst crate

[wolfv]

TODO:

• run precreate code if specified in the menuinst

[wolfv]

Apparently the name can be a dict:

            "name": {
                "target_environment_is_base": "superspyder 1.2 ({{ DISTRIBUTION_NAME }})",
                "target_environment_is_not_base": "superspyder 1.2 ({{ ENV_NAME }})"
            },

[Hofer-Julian]

Linux is now nearly working.

Running cargo run -- create napari-menu gives the following desktop file in .local/share/applications/napari (0.5.4).desktop

[Desktop Entry]
Type=Application
Encoding=UTF-8
Name=napari (0.5.4)
Exec=unset PYTHONHOME && unset PYTHONPATH && /var/home/julian/Projekte/github.com/conda/rattler/.prefix/bin/python -m napari
Terminal=false
Icon=/var/home/julian/Projekte/github.com/conda/rattler/.prefix/Menu/napari.png
Comment=a fast n-dimensional image viewer in Python
Categories=Graphics;Science;
StartupWMClass=napari

Exec doesn't support complicated expressions like this one. We should extract that to a script or trampoline.

[Hofer-Julian]

We went with directly calling the quoted command with bash -c instead.

One thing that still needs to be investigated if we need to run update-desktop-database after installing, my system gives unclear results. We could run it just to be sure. Mimetypes would be also great to have.