Customize: Support instantiation of partials with flat/unwrapped params for...

Customize: Support instantiation of partials with flat/unwrapped params for parity with controls, sections, and panels in [41726].

* Passing `options.params` when constructing `Partial` is now deprecated in favor of just passing `options`.
* Improve usage of jsdoc in JS `Partial` class.
* Also add `defaults` property to `wp.customize.selectiveRefresh.Partial` class for parity with `Control`.

See #42083.

Built from https://develop.svn.wordpress.org/trunk@42037


git-svn-id: https://core.svn.wordpress.org/trunk@41871 1a063a9b-81f0-0310-95a4-ce76da25c4cd