refactor(snippet): implement 'use' subcommand to be more user-friendly

[Akinator31]

This pull request introduces a new way to use snippets as replies in both slash and text commands, along with usability improvements and enhanced localization support. The most significant changes include adding a /snippet use <key> subcommand, allowing !snippet <key> as a shortcut for using a snippet, and updating help text and translations to reflect these new capabilities.

New snippet usage functionality:

• Added a /snippet use <key> subcommand to the slash command, enabling users to quickly use a snippet as a reply in a ticket. This includes backend logic for handling the new subcommand and appropriate user feedback.
• Enabled !snippet <key> (and alias !s <key>) as a text command shortcut for using a snippet, in addition to the existing !reply {key}.

Improvements to argument parsing:

• Refined parsing logic for the create and edit text commands to handle leading whitespace more robustly and extract the snippet key/content more reliably.

Localization and help text updates:

• Updated help and usage messages in both English and French to document the new /snippet use subcommand and the !snippet <key> shortcut, making it easier for users to discover these features.
• Added new translation keys and messages for the new subcommand and its responses in both languages.