[TwigBundle] Created stopwatch tag for profiling templates

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #7953
| License       | MIT
| Doc PR        | symfony/symfony-docs#2630

This PR is the continuation of #7953

This PR adds a new tag to Twig which you can use to time parts of a template and see it in the timing tab of the profiler.

Usage:

````jinja
{% stopwatch foo %}
... some things that gets timed
{% endstopwatch %}
````

The main differences with the original PR are:

* I have removed the code that checks is stopwatch is available as the check is already done during compilation
* I have removed the check for names that was already used as it is not needed (the stopwatch knows how to deal with such cases).