This was always awkward as fish script, and had problems with
interrupting the autoloading.
Note that we still leave the old function intact to facilitate easier
upgrading for now.
Fixes#7145.
This adds a new readline command self-insert-notfirst, which is
analogous to self-insert, except that it does nothing if the cursor
is at the beginning. This will serve as a higher-performance implementation
for stripping leading spaces on paste.