New commands 'completion-preview-insert-{word,sexp}'

New commands 'completion-preview-insert-{word,sexp}'

* lisp/completion-preview.el
(completion-preview-partial-insert): New function.
(completion-preview-insert-word)
(completion-preview-insert-sexp): New commands.
(completion-preview-commands): Add them.
(Commentary): Document them.
(completion-preview-active-mode-map): Add comment.
* test/lisp/completion-preview-tests.el
(completion-preview-insert-calls-exit-function): Update.
(completion-preview-insert-nonsubword)
(completion-preview-insert-subword)
(completion-preview-insert-sexp):  New tests.  (Bug#71716)

(cherry picked from commit b3017e7c252462297ee3887dd6d65cf14f138b85)