Use a dedicated capf instead of rebinding 'C-M-i'

Instead of rebinding 'C-M-i' to a command that inserts the suggested
completion, we make 'completion-at-point' do the right thing when the
preview is active by temporarily adding a prioritized capf.

Thanks to João Távora for suggesting this approach.

* completion-preview.el (completion-preview-active-mode-map): Remove.
(completion-preview--insert):
(completion-preview-insert-function): New functions.
(completion-preview-active-mode): Adjust.

* completion-preview.el (completion-preview-predicate): Rename

Use a dedicated keymap for when the preview active

* completion-preview.el (completion-preview-hide): New function.
(completion-preview-active-mode-map): New keymap.
(completion-preview-active-mode): New minor mode.
(completion-preview--post-command)
(completion-preview-mode)
(completion-preview-show): Adjust.
(completion-preview-insert-or-complete): Remove.
(completion-preview-insert): New command.

Handle non-zero completion base offset

Compatibility definition for 'minibuffer--sort-by-length-alpha'

* (completion-preview-hook): Fix typo in docstring

Expand Elisp commentary

Add Elisp package headers

Add COPYING file

Add an autoload cookie for 'completion-preview-mode'

Initial commit