[VarDumper] polymorphic and stateless output selection
bug #12365 [VarDumper] polymorphic and stateless output selection (nicolas-grekas)

This PR was merged into the 2.6-dev branch.

Discussion
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[VarDumper] polymorphic and stateless output selection

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/4243

While writing the documentation for the VarDumper component, it came to me that selecting the output destination was a state-full process for streams.
This PR fixes that by allowing not only callbacks but also streams on the second argument of `AbstractDumper::dump()`.
I also updated some interfaces to make the feature/names more straightforward.

See https://github.com/symfony/symfony-docs/commit/c578fe79627054a35db556ead758f1ea7f82f172 for the related documentation update.

Commits
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9b9fb5a [VarDumper] polymorphic and stateless output selection