Add support for message reactions and reaction count updates

[Al-Muhandis]

Motivation

• Add first-class support for Telegram message reactions so the library can receive and expose reaction-related updates.
• Surface reaction events to consumers so apps can react to individual reaction changes and aggregated reaction counts.

Description

• Added new data models TTelegramReactionType, TTelegramReactionCount, TTelegramMessageReactionUpdated and TTelegramMessageReactionCountUpdated and corresponding lists in tgtypes.pas to represent reaction payloads and counts.
• Extended TUpdateType with utMessageReaction and utMessageReactionCount, updated UpdateTypeAliases and UpdateTypeClasses, and added MessageReaction / MessageReactionCount properties and getters on TTelegramUpdateObj in tgtypes.pas.
• Implemented JSON parsing constructors/destructors for the new types to build reaction objects from incoming update JSON in tgtypes.pas.
• Exposed new events TMessageReactionEvent and TMessageReactionCountEvent, added fields/properties OnReceiveMessageReaction and OnReceiveMessageReactionCount, implemented DoReceiveMessageReaction and DoReceiveMessageReactionCount handlers, and wired dispatch for the new update kinds in tgsendertypes.pas.