Enhance sc_pm_nextprev with new parameter handling & bring PM_NEXTPREV shortcode to v 2.3.3

[rica-carv]

Refactor sc_pm_nextprev method to support new parameter format and template handling.

Motivation and Context

Allow customization of the netxprev selectors in MP pages...

Description

Changed code to allow more parms to the shortcode (V2.3.3 way), not just a single one (OLD V 0.7 way)

How Has This Been Tested?

Tested in latest github code

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation (a change to man pages or other documentation)

Checklist

• My code adheres to the e107 code style standard.
• I have read the contributing document.
• I have added tests to cover my changes.
• All new and existing tests pass.