Add Environment Variable Editor

[ThomasJRyan]

This PR adds an environment variable editor to the command palette.

A large change has been made to the SharedVariables class to allow differentiation of multiple environment files instead of having them all merging into a single group of variables.

Most of the functionality was a merge of the Help Screen and Headers widget.

Environment file watching has been commented out due to it being annoying when modifying variables in-app. I think I'd rather find a way to keep it and just not have it fire off when changed from Posting

TODO:

• Focus should be given to the EnvironmentTable after an env file is selected in the Select
• A setting should be provided to disable writing to env files
• It'd be nice to be able to edit variables in-place instead of having to remove and add them
• Hotkey bindings
• Ability to create env files for collections (though admittedly I might leave that for a later PR)
• Test that PATH variables work and cannot be written to

Bugs:

• Focus isn't given back to the app when the screen closes (the reset_focus callback isn't being called for some reason). The causes a crash when F1 is pressed immediately after closing the screen