[5.2] Router: Check directly for RouterInterface

[Hackwar]

Summary of Changes

I have no idea why I took this complicated approach of using Reflection to see if the given object implements the RouterInterface. This PR changes the code to be simpler and to just use the instanceof operator.

Testing Instructions

Codereview

Link to documentations

Please select:

• Documentation link for docs.joomla.org:

• No documentation changes for docs.joomla.org needed

• Pull Request link for manual.joomla.org:

• No documentation changes for manual.joomla.org needed

[Fedik]

I have tested this item ✅ successfully on 062d2f9

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[Hackwar]

For 6.0 we should enforce the interface by typehinting the input vars properly.

[wilsonge]

Trivial change so merging with a single good test + my code review