Add script to report stuff missing from help

Initial version just reports missing stuff, future changes should do more checking like:

* Check default key bindings
* Check default parameter values
Add Windows mode bindings for character searching

F3/Shift+F3 are now bound to character searching in Windows mode.
This roughly matches Visual Studio bindings.

Note that traditionally F3 copies text from previous line starting
using the position of the cursor in the current line.  It's not
hard to implement, but I'm not sure it's a useful feature, especially
with YankLastArg (which I could see mapping to F3 but probably won't.)
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