Add validateReaction Helper Function

[Silent-Watcher]

Feature: Export validateReaction Helper

Description:
Export the internal validateReaction function (which normalizes and checks against the whitelist) so users can validate reaction strings in their own code.

Acceptance Criteria:

• Function validateReaction(reaction: string) is exported from the package.
• It returns a normalized version of the reaction if valid.
• It throws an error with a clear message if the reaction is not allowed.
• The function respects the plugin's case sensitivity and whitelist options.
• Tests are added for this exported function.
• Add usage examples in docs