Created stopwatch tag
fixed CS
removed unneeded safeguard as it's already done during compilation
removed code that prevents the stopwatch to work properly
change the stopwatch argument to be any valid expression
merged branch fabpot/twig-stopwatch-helper (PR #8719)

This PR was merged into the master branch.

Discussion
----------

[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 %}
````

Commits
-------

29a58e7 change the stopwatch argument to be any valid expression
4590974 removed code that prevents the stopwatch to work properly
2f67776 removed unneeded safeguard as it's already done during compilation
bbad387 fixed CS
f39ed57 Created stopwatch tag