Add a method to register custom message providers

[utarwyn]

Description

This PR adds a method to register a message provider programmatically.
You can use it to display dynamic values that may change over time.

Few examples:

• Display a dynamic bet to play a game
• Display the date/hour
• Display your own statistics

ℹ️ You can use the default variables in the provided messages

Usage example

You can use it this way:

const bot = new TicTacToe({ language: 'en' });
bot.addMessageProvider('game.title', () => 'Current date is ' + new Date());

Signature of the method (in TypeScript) is:

/**
 * Adds a message provider for a given key.
 * Key must exist, otherwise an error will be thrown.
 *
 * @param key key corresponding to the added provider
 * @param provider function that dynamically supplies the message
 */
addMessageProvider(key: string, provider: () => string): void;

🌂Closes #258
🚀 Will be available from v3.x.

[sonarcloud]

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