[Console] added a Process helper
[Console] Add process helper tests
feature #10627 [Console] added a Process helper (fabpot, romainneutron)

This PR was merged into the 2.3-dev branch.

Discussion
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[Console] added a Process helper

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | https://github.com/symfony/symfony-docs/pull/3930

This PR replaces #10609.
I've fixed minor errors, added tests and allowed to pass an array of arguments to escape as a command

Commits
-------

edc1bfe [Console] Add process helper tests
faffe7e [Console] added a Process helper
[Debug] Symfony debug extension
[Debug] a README for the debug extension
[VarDumper] algo to clone any PHP variable to a breadth-first queue
[VarDumper] symfony_debug ext. fast and memory efficient cloning algo
[VarDumper] add casters for per class/resource custom state extraction
[VarDumper] casters for exceptions representation
[VarDumper] casters for SPL data structures
[VarDumper] Add workaround to https://bugs.php.net/65967
[VarDumper] casters for PDO related objects
[VarDumper] casters for removing the entity manager from Doctrine proxies
[VarDumper] interface for dumping collected variables
[VarDumper] CLI dedicated dumper and related abstract
[VarDumper] HTML variant of the CLI dumper
[VarDumper] spec for json based representation of PHP vars
[VarDumper] json dumper for interoperable displaying
[VarDumper] tests for JsonDumper
[VarDumper] tests for CliDumper