Tags implementation

[gdude2002]

This branch introduces a new extension, and also a new subsystem.

1. Tags extension — ??tag on Discord will retrieve the content of a tag stored in the tags/ directory of the community repo.
2. Git scaffolding, which makes it a bit easier to handle Git repo stuff if we ever need it for another extension.

Additionally, the tags parsing logic is placed within a subproject, and that subproject can be built and run in order to validate a directory of tags.