[Translation] Added template for relative file paths
Added a ConcreteDumper
Changed placeholders
feature #9852 [Translation] Added template for relative file paths in FileDumper (florianv)

This PR was merged into the 2.5-dev branch.

Discussion
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[Translation] Added template for relative file paths in FileDumper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | https://github.com/symfony/symfony/issues/9845
| License       | MIT
| Doc PR        |

Added ability to support templates for relative paths to translation files.
The file dumpers will try to create the directories if not existing.
Also made `IcuResFileDumper` extending `FileDumper`.

Commits
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a04175e Changed placeholders
623d149 Added a ConcreteDumper
84f0902 [Translation] Added template for relative file paths