This lifts out the GitHub callout transformer from
`modules/markup/markdown/goldmark.go` to `callout/github.go`.
While there, clean up the transformer code:
- Use a map to look up supported callout types, rather than a regexp.
- Allow the callout type to be in any case, rather than just uppercase.
- Simplified `.Segment.Value()` to `.Text()`.
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
