made twig extension service private
merged branch kriswallsmith/master (PR #6842)

This PR was merged into the master branch.

Commits
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731cd49 made twig extension service private

Discussion
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[TwigBundle] made extension a private service

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~