Added Stopwatch Helper
merged branch WouterJ/stopwatch_php (PR #8968)

This PR was merged into the master branch.

Discussion
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Added Stopwatch helper for PHP templates

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | symfony/symfony-docs#2960

Commits
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3ee2989 Added Stopwatch Helper