Everytime you hit ctrl+s in VS code, the whole project in checked, which
normally is a nice thing, and I would not like to disable that in my
configuration.
But in rinja, the diffent integration crates don't work well with one
another and every single one will have compile errors in the automatic
check, because `rinja_derive` won't know what the relevant `rinja_xyz`
crate is. This renders the automatic check useless. Then it takes ages
to compile and recompile all transitively used `proc_macros`.
This PR moves all integration crates in there own, individual
workspaces. `cargo check` will ignore them unless you traverse manually
into their folders. The CI is set up to do just that.