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Fixes #8468

This change enables UP-arrow and DOWN-arrow full-line history completion.
For example, if you write `git clone` and press UP:

- Before this change,
  it will use the last command that starts with `git`.

- After this change,
  it will use the last command that starts with `git clone`.

Since e537ee9d, the reverse-menu-complete keybinding has no longer been
properly bound (it was accidentally bound to the delete key).

This commit again binds it to shift-tab.

Delete key (of Fn+Delete on Mac) would print "~" instead of deletion.

Aligned comments

Esc+. works as "last arg" on both bash and zsh. Seems like a shame to introduce a new standard.

Conflicts:
	lib/key-bindings.zsh

Conflicts:
	lib/key-bindings.zsh

Esc+. works as "last arg" on both bash and zsh. Seems like a shame to introduce a new standard.

This binding doesn't work when the edit-command-line.zsh file is loaded
after the key-bindings.zsh file because 'bindkey -e' in key-bindings.zsh
resets the binding. Moving the bindings to they key-bindings.zsh file
and removing edit-command-line.zsh.

Moving all zsh config options into a lib/ subdirectory to make way for some upcoming changes to directory structure and configuration options