helix: add keybind to run python files

I'm tempted to wait for #61 to implement this, given the amount of work
that's gone into it, but I can't reject a genuinely easy solution. I
still believe that doing #61 will be better in the long term. That's
definitely coping, but let me cope, okay? Just check the date on this
commit message and you'll understand.

apps/cli/helix/keybinds.nix

@@ -12,6 +12,7 @@ let
     # Select next word, with no spaces selected at all. Great for replacing via `q+c`
     q = "@emiw"; # Uses macros, added in unstable helix version
     A-q = "@emiW";
+    space.p = "@:sh python <C-r>%<ret>";
 
     space.x = ":toggle whitespace.render all none"; # ` x` shows whitespace