Add __fish_anypython helper function

This just finds the first usable python and echos it, so it can then
be used.

We have a few places where we use it and I'm about to add some more.

share/functions/__fish_anypython.fish

@@ -0,0 +1,10 @@
+function __fish_anypython
+    # Try python3 first, because that's usually faster and generally nicer.
+	for py in python3 python2 python
+        command -sq $py
+        and echo $py
+        and return 0
+    end
+    # We have no python.
+    return 1
+end