refactor: rename format_any_of to format_alternatives.

* specl/matchers.lua (Matcher.format_any_of): Rename from this...
(Matcher.format_alternatives): ...to this. Adjust all callers.
* docs/specl.md (Custom Matchers): Update.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>
specl: support new `any_of` matcher adaptor.

* specs/matchers_spec.yaml, specs/shell_spec.yaml: Examples of
new behaviours with `any_of` adaptor.
* specl/matchers.lua (concat): Add a new `adaptor` parameter to
decide between formatting a list with a final `or` or `and`
depending on whether called from `any_of` or `all_of`.  Adjust
all callers.
(reformat): Likewise.
(Matcher): Add a new `all_of?` receiver.
(alternatives_msg): Factor out common code from `any_of?` and
`all_of?` receivers.
* NEWS: Update.

Signed-off-by: Gary V. Vaughan <gary@gnu.org>