feat(nix-compat): allow parsing and emitting log lines

This adds code allowing to parse (and emit) the "internal-json" log
format that Nixcpp emits.

Move the Verbosity struct from worker_protocol.rs out into
src/log/mod.rs, it's not only used there.

Change-Id: Ia35e739c83e06080a677b576616bdd0d4a821cf4
Reviewed-on: https://cl.tvl.fyi/c/depot/+/13167
Autosubmit: flokli <flokli@flokli.de>
Tested-by: BuildkiteCI
Reviewed-by: Brian Olsen <me@griff.name>
Reviewed-by: edef <edef@edef.eu>

users/edef/weave/Cargo.nix

@@ -2987,10 +2987,6 @@ rec {
             name = "mimalloc";
             packageId = "mimalloc";
           }
-          {
-            name = "serde_json";
-            packageId = "serde_json";
-          }
         ];
         features = {
           "async" = [ "tokio" ];