Implement immutable cache headers

[programmin]

When you have js files that will not change in a certain version of Wordpress, why not set the immutable cache that is enabled on various browsers?

​https://hacks.mozilla.org/2017/01/using-immutable-caching-to-speed-up-the-web/

For example this url should never change output unless user changed the wordpress version, in which case the ver= will change, forcing browser to load that url.

/wp-admin/load-scripts.php?c=gzip&load%5B%5D=jquery-core,jquery-migrate,jquery-ui-core,jquery-ui-widget,jquery-ui-mouse,underscore,mediaelement,jquery-ui-sortable&ver=4.7.4

[dd32]

I have no issue setting this flag, I like the suggestion after reading a little about it.

I think the only scenario we should set it though, is in the case where $_GET['ver'] is present and it does not contain a - character (in the ver param) to avoid caching issues while testing beta's, RC's, and primarily nightlies (where in, the ver may not be unique if operating from the /src directory)

[ocean90]

Previously: #28722