[TwigBundle] Moved the setting of the default escaping strategy from the Twig engine to the Twig environment

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #11773
| License       | MIT
| Doc PR        | n/a

The default escaping strategy was set on the Twig Engine and not the Twig Environment. That's a problem under some circumstances (like what #11773 describes), but it's also much better to set everything on the Twig  Environment directly.

What about supporting ``.css.twig`` too ? Twig supports the css escaping context too.