[2.3] [FrameworkBundle] [Templating] added Stopwatch support to the PHP engine

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

I did not include tests nor documentation because there weren't any for TimedTwigEngine (I took it as an example). If I'm mistaken and they are needed, I'll gladly write them.