Expose line editor state to script

[xiaq]

No description provided.

[xiaq]

The direct motivation is that I want to bind E in navigation mode to invoke $EDITOR on the selected file, and I feel strongly that this should be done in pure elvish, instead of being implemented as yet another builtin. Also, it would be nice to modify the binding as if it is an elvish table. What I want to achieve should be doable with something like this:

le:binding[navigation][E] = { $env:EDITOR $le:selected-file }

[xiaq]

le:binding[navigation][Ctrl-E]={
  le:current-command=$E:EDITOR' '$le:selected-file
  le:return-line
}