Don't reset the parser to insertMap for field editor.

This is because the field editor isn’t necessarily going back
to the insert map. In particular, the ex field’s field editor
goes back to the ex command map.
Add exCommandCompletionMap.

Before, we were using the regular completion map in the
field editor; however, we need a different map for completion
in the ex command field, since the base keystrokes are
different.

Right now this is identical to the standard completion map
except that it includes the ex command map instead of the
insert map. A couple of attempts to create a base
completion map with the completion keystrokes and then
include that in addition to either the ex map or the insertion
map depending on which completion map we were in didn’t
seem to go so well, so we’ll move on for now.
Set up the ex completion map correctly on each keystroke.

Whenever a keystroke is successfully mapped, the parser
resets, so we need to be ready in cases where the field
editor is showing a completion window to reset the parser
map.