Ensure parity between JS and PHP calculation of theme customizer control IDs

[westonruter]

Do global search/replace for customize control selector

In WP_Customize_Conrol::render(), the $id for a control is constructed via:

      $id    = 'customize-control-' . str_replace( '[', '-', str_replace( ']', '', $this->id ) );

The PHP str_replace function does a global replacement. In JavaScript, however, if you call the string.replace() with a string as the pattern argument instead of a RegExp, it will do a single replacement instead of a global one. Therefore, RegExp literals with the g flag must be supplied to ensure that the ID calculated in JS matches the ID calculated in PHP.

If a setting is registered for a multidimensional array, for example myplugin[foo][bar][baz], then the field in the Customizer will fail to pick up changes because it is not able to find the control's container element via the selector.

[westonruter]

Use RegExp with global flag instead of strings for replacement patterns

[westonruter]

Just wanted to mention that the patch for this bug is a dependency for the new Widget Customizer plugin: ​https://github.com/x-team/wp-widget-customizer

[nacin]

In 25680:

Ensure we globally replace [ and ] in customizer control IDs, to match the PHP.

props westonruter.
fixes #25238.