Change config option 'default_suffix' to 'extension' to match other renderers

[Xerkus]

Provide a narrative description of what you are trying to accomplish:

This PR changes config option for resolver default suffix from default_suffix to extension to match plates and twig template renderers. Related: #64

• [x] Are you creating a new feature?
  • [x] Why is the new feature needed? What purpose does it serve?

    64 introduced a way to configure default suffix using configuration option $config['templates']['default_suffix'] however two other template renderers provided by framework use key extension

    This PR fixes inconsistency inadvertently introduced by #64 by making use of extension key.

  • [x] How will users use the new feature? By providing alternative suffix in config:

    [
    'templates' => [
        'extension' => 'php',
    ],
    ]

  • [x] Base your feature on the develop branch, and submit against that branch.
  • [x] Add only one feature per pull request; split multiple features over multiple pull requests
  • [x] Add tests for the new feature.
  • [ ] Add documentation for the new feature. Documentation needs to be added in another repository
  • [x] Add a CHANGELOG.md entry for the new feature.

[froschdesign]

@Xerkus I can not found any documentation for this configuration option for zend-view. Only for Plates and Twig:

• https://docs.zendframework.com/zend-expressive/v3/features/template/plates/#configuration-and-factory
• https://docs.zendframework.com/zend-expressive/v3/features/template/twig/#configuration

It would be great if you could add this.

[Xerkus]

@froschdesign i didn't add it yet, I discovered inconsistency when I was preparing a PR for the docs

[froschdesign]

@Xerkus

…I was preparing a PR for the docs

Great! 👍