Use PSR-3 LoggerInterface to allow custom logging implementations.

[noplanman]

As suggested in #831 this PR implements the PSR-3 LoggerInterface.
(@garak I'd appreciate your feedback as this is new-land for me)

It is still 100% backwards compatible (spewing deprecation notices) but the deprecated code will be removed in the near future.
For dev purposes, I'll move the Monolog requirement to require-dev when the deprecated code gets removed and the new logging methodology is enforced.

New way of initialising the logger (using Monolog in this example):
First we need to get Monolog via Composer:

composer require monolog/monolog

Then we initialise the logger objects in our webhook or getUpdates script:

use Monolog\Formatter\LineFormatter;
use Monolog\Handler\StreamHandler;
use Monolog\Logger;
use Longman\TelegramBot\TelegramLog;

TelegramLog::initialize(
    // Main logger that handles all 'error' and 'debug' logs.
    new Logger('telegram_bot', [
        (new StreamHandler(__DIR__ . "/logs/{$bot_username}_debug.log", Logger::DEBUG))->setFormatter(new LineFormatter(null, null, true)),
        (new StreamHandler(__DIR__ . "/logs/{$bot_username}_error.log", Logger::ERROR))->setFormatter(new LineFormatter(null, null, true)),
    ]),
    // Updates logger for raw updates.
    new Logger('telegram_bot_updates', [
        (new StreamHandler(__DIR__ . "/logs/{$bot_username}_update.log", Logger::INFO))->setFormatter(new LineFormatter('%message%' . PHP_EOL)),
    ])
);

[akalongman]

I likw this PR 😍 Use PSR's is the right way