Value and type parameter packs. (apple#1956)

* Add a draft proposal for parameter packs.

* Fix the type of postfix ... operator in the ambiguity example.

* update pitch (#1)

* Add missing SE proposal information.

* Replace the '...' syntax with 'repeat each'.

* Add more introductory explanation to the proposed solution.

* Update proposals/NNNN-parameter-packs.md

Co-authored-by: Remy Demarest <remy.demarest@gmail.com>

* Address editorial feedback

* Add pack iteration and pack element projection to the future directions.

* Move local value packs and add explicit pack syntax to future directions.

* Describe single-element tuple unwrapping under pack substitution.

* Simplify the description of same-type requirements involving parameter packs.

* Update proposals/NNNN-parameter-packs.md

Co-authored-by: Becca Royal-Gordon <beccadax@apple.com>

* Move the introduction above the table of contents.

* Specify trailing closure matching rules for parameter packs.

* Replace the term "type sequence" with "type list" to avoid confusion with the
Sequence protocol.

* Add more commentary on the `repeat each` syntax design.

* Minor editorial changes

* Describe how variadic generic functions interact with overload resolution.

* Update pitch links.

* Update 0393-parameter-packs.md

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Co-authored-by: Slava Pestov <sviatoslav.pestov@gmail.com>
Co-authored-by: Remy Demarest <remy.demarest@gmail.com>
Co-authored-by: Becca Royal-Gordon <beccadax@apple.com>
Co-authored-by: Xiaodi Wu <13952+xwu@users.noreply.github.com>