Adds the inspector commands to lispkit

Keybinds:

- `C-x-i-o`: opens inspector
- `C-x-i-c`: closes inspector
- `C-x-i-a`: attaches inspector
- `C-x-i-d`: detaches inspector

`C-x-i` was chosen because "inspector".
I wonder, does this actually work if enable-developer-extras is false
initially?
Won't the `inspector` you're passing along still be null, seeing as you're
requesting it before setting the option.