registration: Add development requirements to extension.json

registration: Add development requirements to extension.json

Extensions can specify development dependencies in extension.json under
the "dev-requires" key. It's identical to the "requires" field.

Any requirement that is needed to pass tests, including but not limited
to, PHPUnit, QUnit, structure, phan, should be documented in this new
field. The main intention is that CI will ensure that all of these
dependencies are satisfied before running tests.

At standard runtime, the development requirements will be ignored by
MediaWiki, since it only checks for real requirements. Scripts can
manually check development requirements by calling
ExtensionRegistry::setCheckDevRequires( true ) before trying to load
things.

If both "requires" and "dev-requires" are present, MediaWiki will merge
the two together, so the environment will need to satisfy both before
proceeding.

Bug: T193824
Change-Id: I9b2936666ee3c96f5c976c7a17f11c437c2c7f48