Environment Modules: Run `purge` before `load`

Today, an R or Python session that uses Environment Modules always starts by doing module load. However, it should probably be doing a module purge first.

Paraphrasing:

When switching between module based sessions, Positron is not doing ml purge && ml load new_module, it is simply doing ml load new_module. This means that the new env is just updating the list of modules the “new_module” needs, and sometimes, depending on the module configuration, some leftover modules are left in the env... Make sure that prior to a ml load there’s always a ml purge before.

Of course, a downside to this approach is that if you have modules loading in your shell startup scripts, purging will unload them which might not be what you want. We could make the purge optional; on by default but you can disable via:

{
    "positron.environmentModules.environments": {
        "R 4 Stable": {
            "languages": ["r"],
            "modules": ["R/4.4.2", "other/version"],
            "purge": false
        },
    }
}

Start by locating the Environment Modules handling and the environment configuration schema or entry point, then trace where module load is invoked for R and Python sessions. Done means purge runs before load by default, while the per-environment purge setting can disable it; verify the behavior with the relevant existing checks or session startup flow.