Add D&D-style modifier and multi-roll support to /roll

[Dredgerstrad]

Is your feature request related to a problem? Please describe.
The current /roll command doesn't support dice notation modifiers or multi-roll sets, limiting its usefulness for tabletop RPG-style gameplay.

Describe the solution you'd like
Extend /roll to support standard dice notation, including:

• Modifiers — e.g. /roll 1d20+5 adds 5 to the result
• Multiple dice — e.g. /roll 2d6 rolls two six-sided dice and sums them
• Combined — e.g. /roll 3d8+3 rolls three d8s and adds 3

This would bring the command in line with standard D&D / TTRPG conventions and make it far more useful for roleplay scenarios.