Add environment variable dialog

[joseprl89]

Hi 👋

I noticed in your roadmap a feature that should be a good low hanging fruit to pick up and decided to implement it:

Viewing the currently loaded environment keys/values in a popup Environment

Disclaimer, Python is not my main area of expertise, and this is the first time interacting w Textual, so please don't hold back and let me know what I did wrong on the code, happy to address when time allows 👍

Change

• Introduced a command to show under the ctrl+p dialog

• Upon selecting the option, introduced a very simple dialog that just has a list of key value pairs

Not in scope

I decide to keep it simple and not include in the scope:

• Updating environment variables
• Live refresh of the values displayed - if you update the .env file, you'll have to leave the dialog and enter into it again