Fix default parameters in controls.js JSDoc.

[herregroen]

While JSDoc technically supports the @param {string=} optional or @param {string=} [optional] style of setting optional parameters.

It does unfortunately not support @param {string=default} optional or @param {string=default} [optional] style of default parameters.

Instead, as the JSDoc standards recommend, the following should be used:
@param {string} [optional] or @param {string} [optional=default].

src/js/_enqueues/wp/customize/controls.js still contains several notations of the incorrect style.

[atimmer]

In 43391:

Docs: Fix default values for customize/controls.js.

The old values cannot be parsed by JSDoc.

Props herregroen.
Fixes #44520.