Implement activation scripts in intershell-language

[wolfv]

Currently, activation scripts have to be written per-shell. This is quite fragile and difficult, as shells have some slightly different semantics, and one is seldomly an expert on many shells. Also, support for e.g. powershell is quite weak (in conda-forge).

If we have a high-level language for shell operations we could make life easier for developers.

This coudl be a joint effort with the AShT project (http://github.com/regro/asht Abstract Shell Tree), and I started some experiments at https://github.com/wolfv/multisheller