[WIP] [Form] Created FormErrorCollection

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | yes
| Deprecations? | not yet
| Tests pass?   | yes
| Fixed tickets | #7205
| License       | MIT
| Doc PR        | tbd

The only BC break in this PR is checking it with is_array. Iterating
over the array works just the same as previously:

- Normally iterating over the `RecursiveIterator` results in only
  iterating over the errors of the current form.
- Iterating using `RecursiveIteratorIterator` results in iterating over
  all errors recursively.

There is only one thing to do: Adding `__toString()` support and
deprecating `Form::getErrorsAsString()`.