Added '@@' escaping strategy for YamlFileLoader and YamlDumper

Added the possibility to to use '@@' as an escaping strategy for
parameters that should be treated as strings but start with '@'
(i.e. safe mailer passwords).
merged branch tPl0ch/feature/yaml-parameter-escape (PR #7357)

This PR was submitted for the master branch but it was merged into the 2.1 branch instead (closes #7357).

Discussion
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[DependencyInjection] Added @@ escaping strategy for YamlFileLoader and YamlDumper

Added the possibility to to use ```@@``` as an escaping strategy for
parameters that should be treated as strings but start with ```@```
(i.e. safe mailer passwords).

This PR is deprecating https://github.com/symfony/symfony/pull/7307, since as a new feature it has to be compared against the master branch.

| Q | A |
| ------ | ------ |
|  Bug fix?  |  yes  |
|  New feature?  |  yes  |
|  BC breaks?  |  no  |
|  Deprecations?  |  no  |
|  Tests pass?  |  yes  |
|  Fixed tickets  |  #4889  |

Commits
-------

8cdf387 [DependencyInjection] Added '@@' escaping strategy for YamlFileLoader and YamlDumper