resoureloader: Consolidate styles-only queue at the top

resoureloader: Consolidate styles-only queue at the top

This effectively reverts d6b4d3c537 and declines T97420.
This was previously attempted in b7c0e537eb.

Drop support for position "bottom" for addModuleStyles().
This feature was only recently introduced with the intent
to optimise page load performance, but had an adverse effect.

It increases chances of FOUC due to late discovery of these styles.
It also caused minor problems for some gadgets and extensions
that did not or were unable to set these flags. Some mobile
code was introduced around the same time as this feature and
was never given position=top.

Stylesheets that don't affect initial render or are only needed
on interaction should be loaded via addModules() instead; which
is handled by the asynchronous loader in JavaScript.

Change-Id: Ib9821b7b87cfc8a59062bb6ca358974fdb01ced1