[TwigBundle] Created stopwatch tag for profiling templates

[fabpot]

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:

{% stopwatch foo %}
... some things that gets timed
{% endstopwatch %}

[fabpot]

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).

[fabpot]

I've made further modifications:

• The stopwatch name can now be any valid expression (more consistent with the other Twig built-in tags)
• I've removed the possibility to add the name after endstopwatch (because of the previous change and also for more consistency)