Inherit text props in Completion Preview insertion commands

Inherit text props in Completion Preview insertion commands

When completing with Completion Preview mode commands,
inherit text properties from surrounding text similarly to
other completion commands, such as 'completion-at-point'.

* lisp/completion-preview.el (completion-preview-insert)
(completion-preview-partial-insert)
(completion-preview-complete): Use 'insert-and-inherit'
instead of 'insert' to insert text for completion.
* test/lisp/completion-preview-tests.el
(completion-preview-insert-inherits-text-properties): New
test.