Expand abbr explicitly (#5762)

* Add "expand-abbr" bind function

This can be used to explictly allow expanding abbreviations.

* Make expanding abbr explicit

NOTE: This accepts them for space only, we currently also do it for \n
and \r.

* Remove now dead code

We no longer trigger an abbr implicitly, so we can remove the code
that does it.

* Fix comment

[ci skip]

sphinx_doc_src/cmds/bind.rst

@@ -112,6 +112,8 @@ The following special input functions are available:
 
 - ``end-selection``, end selecting text
 
+- ``expand-abbr`` expands any abbreviation currently under the cursor
+
 - ``forward-bigword``, move one whitespace-delimited word to the right
 
 - ``forward-char``, move one character to the right