Create JHtml::_('behavior.core') to decouble core.js from mootools framework

### Issue
Currently, when we want to load `core.js` (kind of our own Joomla JavaScript "library") we use the same function as when we want to load the MooTools JavaScript library. That is `JHtml::_('behavior.framework')`.
So far this wasn't a problem since core.js needed MooTools anyway.
Since we want to remove MooTools and there are quite a few PRs around which deals with that and also rewrites core.js to jQuery, we may end up loading MooTools only to get core.js. This is of course not good.

### Proposed Solution
This PR will introduce a new function `JHtml::_('behavior.core')` which is supposed to only load core.js.
Currently, this would still load MooTools since core.js depends on it. It's implemented as a simple proxy to behavior.framework for now.
As soon as core.js is rewritten to jQuery, the function can be changed to load jquery.framework and core.js instead.
I already wrote the code for that with comments what needs to be done.

### Goal
Introducing this new class already "ahead of time" would allow us to start rewriting our extensions to use this new class, so when core.js is changed, we don't need to change anything anymore.
After all core javascript functions are rewritten to jQuery, we can then deprecate behavior.framework and remove it with J4.0.
This also gives 3rd party developers a bit more time to do the transition.