Bug 1725125 - Make a hybrid build system combining non-unified with unified. CLOSED TREE

By hybrid unified system we understand a system that encapsulates modules that are built in the unified mode
but also other modules, like `dom/Animation`, as an example, in the non unified environment.

This approach is desirable since we already have most of the modules transitioned to the non unified system but there are
still some that are not yet compatible, but in the long term this will be done by each module owner and can be also tested
locally using the build system.

If a module can't be built outside the unified method it's `moz.build` config file needs to have `REQUIRES_UNIFIED_BUILD = False`

To also enable this we need to have a flag from `mozconfig`, like:

```
ac_add_options --disable-unified-build
```

Differential Revision: https://phabricator.services.mozilla.com/D122328