[Form] make prototype_name available in twig template

When using several collection types on the same page you might wan't to use different prototype names. Having the name in the template allows for a clean javascript to replace the prototype name in the prototype (see adjusted sample in the Doc PR). 

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| 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/2544