Complete shorthands to longhands for symbol-completing tables

Complete shorthands to longhands for symbol-completing tables

Shorthands aren't symbols, they're text forms that 'read' into
symbols.  As such, shorthands aren't candidates in these tables of
symbols.  But in some situations, if no other candidates match the
pattern, we can e.g. complete "x-foo" to "xavier-foo" if the shorthand

  (("x-" . "xavier-"))

is set up in the buffer of origin.

bug#50959

* lisp/help-fns.el (help--symbol-completion-table): Report
`symbol-help' category.

* lisp/minibuffer.el (completion-styles-alist): New 'shorthand'
style.
(completion-category-defaults): Link 'symbol-help' category with
'shorthand' style.
(minibuffer--original-buffer): New variable.
(completing-read-default): Setup minibuffer--original-buffer.
(completion-shorthand-try-completion)
(completion-shorthand-all-completions): New helpers.