cmd: add support for ENV command

[Delta456]

Implements #419

Output examples/demo.gif
Env PROMPT "〉"

Require echo

Set Shell "bash"
Set FontSize 32
Set Width 1200
Set Height 600

Type "echo 'Welcome to VHS!'" Sleep 500ms  Enter

Sleep 5s

and also allows adding environment variables.

Env TESTING "this works"

Type "echo $TESTING"
Enter
Sleep 3s

[Delta456]

Ready for review

[maaslalani]

Awesome work @Delta456, thank you so much!

[rahji]

Can we get a new Release so this feature is included when someone does go install github.com/charmbracelet/vhs@latest? PS the helix editor's syntax highlighting file will need updating.

[SVendittelli]

I second the request for a new release. I install using winget and I'd like to be able to use the new Env functionality.