Bug 1840781 - [css-nesting] Enable by default. r=boris

After the pending test fixes (bug 1843349) we only fail tests due to two
reasons:

 * :has() being unsupported. Not related to the feature itself.

 * The nesting selector being inside another (invalid) forgiving
   selector, e.g. :is(!&) not being preserved.

The later is pretty much in edge-case-land, and I think
w3c/csswg-drafts#8356 would be more consistent
rather than doing it only when an & is found (which is very very weird).

So I wouldn't want to block on that. Let's let it ride the trains.

Differential Revision: https://phabricator.services.mozilla.com/D183519

UltraBlame original commit: 88d3fbc7589e6b57adc0946fe197ada285ac84b7

modules/libpref/init/StaticPrefList.yaml

@@ -8257,7 +8257,7 @@
 # Whether CSS nesting is enabled.
 - name: layout.css.nesting.enabled
   type: RelaxedAtomicBool
-  value: @IS_NIGHTLY_BUILD@
+  value: true
   mirror: always
   rust: true