[Godot 4] Highlight editor terminal border on focus

[lihop]

Draw an outline around the editor terminal when it is focused.

Although this feature is currently lacking in GodotXterm, the bright blue border of Godot 4's default theme makes the lack of this feature more obvious.

Implementing #29 would also help indicate that the editor terminal is or isn't focused.

[lihop]

Closing in favour of only having a hollow cursor (#29) as this is implemented in #98.
The hollow cursor only behaviour is consistent with the script editor and VSCode.
I tried adding a bright blue border, but found it overly distracting.