We can't use #[from] on Error::Msg(String) because String does not implement Error.
(Which it shouldn't; see e.g. https://internals.rust-lang.org/t/impl-error-for-string/8881.)
So we implement From manually for Error::Msg, since our current code was written
in that way for error-chain.
Move code to build assets to its own file. That results in better modularity and flexibility.
It also allows us to simplify HighlightingAssets a lot, since it will now always
be initialized with a SerializedSyntaxSet.