refactor(3p/nix/libexpr): Extract static helpers out of parser.y

Moves several of the static helper functions into a new parser.cc
file.

Once the rest of the code is usefully extracted, these will be moved
to a private namespace.

Change-Id: I0d7b53dcefe31bb5c6bad3ad7f5fcb48276bf799
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1218
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/CMakeLists.txt

@@ -59,6 +59,7 @@ target_sources(nixexpr
     json-to-value.cc
     names.cc
     nixexpr.cc
+    parser.cc
     primops.cc
     symbol-table.cc
     value-to-json.cc