Add tracked_path with feature gate

[tristandruyen]

As the build.rs is quite bad ergonamically let's try to fix this OOTB for nightly users.

Refs:

• Tracking Issue for proc_macro::{tracked_env, tracked_path} rust-lang/rust#99515
• Picking up new router files #2
• proc_macro::tracked_path

Todos:

• use tracked_path if feature is avail
• update docs to mention build script is only needed on non-nighly compilers (until tracked_path is stabilized)
• Publish new version