Upstream Mooncake's `BBCode` to Julia's Compiler

[yebai]

Precedence for extra Compiler tools: JuliaLang/julia#57193, which is similar to BBCode perhaps.

[willtebbutt]

At the very least we can carve it out into a separate package in the Mooncake repo. We really need to make a clear case to various people that this is something useful before we attempt to upstream, and we don't currently have the evidence for this in an easily-digestible form (docs + worked examples are needed).

edit: the reason to at least carve it out into a separate package in the Mooncake repo is to ensure that e.g. Libtask can depend on it without having to depend on the entirety of Mooncake.