drupal_add_js() is missing the 'browsers' option

Problem

• CSS can be conditionally loaded for limited 'browsers' (e.g., IE) only, but JS cannot.
• #1077878: Add HTML5shiv to core → cannot be done without conditional comment support for JS.
• Modules like No IE6 cannot load JavaScript files for specific browsers.

Proposed solution

• Make it consistent with drupal_add_css
• Hacking a #browsers condition into the current code of drupal_get_js() doesn't really make sense.
• Instead, further align drupal_get_js() with drupal_get_css(), as originally proposed in #330082-20: Refactor drupal_get_js() to use render layer similar to drupal_get_css() → already.
• Introduce #type 'scripts' -- equivalent to existing #type 'styles'.
• As already the case for #type 'styles' / drupal_get_css(), properly separate the current processing of JS into a #group_callback and #aggregate_callback.

  Side-benefit: Default behavior can be changed and improved in contrib.

• Only do the necessary to align #type 'scripts' with 'styles', and to introduce support for 'browsers'.

  A major long-term benefit of doing so is that we're finally able to abstract and merge further parts of the #group_callback and #aggregate_callback logic into common helper functions -- paving the way to get to the long-standing idea of having a single generalized system for "support files"/"page requisites" in Drupal core.

Current State

The last patch submitted #136 has been RTBC.
This patch does everything proposed. It also passed all the JS test, and a couple of new test were added.

Seems good to go.