UI: Add Environment Selection and Modal

[SoulKa]

Is your feature request related to a problem? Please describe.
We need a UI concept and implementation for Environments.

Describe the solution you'd like

• A selection where I can add or select an environment (likely a dropdown)
• A modal where I can edit the variables of an environment (can reuse VariableTab component)

Additional context

• The environments can be found under collection.environments at the collection store