[Feature Request] Add support for Stage channels.

**Is your feature request related to a problem? Please describe.**
Bot cannot join the newly released stage channels.

**Describe the solution you'd like**
Add support for stage channels.

**Additional context**
None

Start by locating the bot's Discord channel-handling entry points and reviewing how the project currently identifies supported channel types. Check the Discord/JDA API documentation for stage channels, then determine the expected behavior for joining and handling them. Done means the bot can join a stage channel without treating it as unsupported, with coverage for the relevant behavior if the project has tests.

Written by the indexing model from the issue text.