Reduce VRL core such that it doesn't have any other dependencies to VRL

[pablosichert]

As part of the LLVM work in #10518, a library with basic VRL functionality as precompiled LLVM bitcode needs to be built.

Currently, including Value and Target from vrl_core has a dependency on the VRL parser and the compiler, making it large in binary size.

The goal is to strip down VRL core such that other parts of VRL can depend on it, but it depends on none of those.