refactor(3p/nix/libexpr): Extract parser header out of Yacc file

First step (of many?) towards extracting all the inline code from the
Yacc file and keeping it somewhere more accessible instead.

Note that none of this code has previously been touched by a linter or
formatter, pretty much ever, so as it is extracted it also undergoes
similar changes to the whole codebase after the initial fork.

Change-Id: If3b7181f22e3b3fd8c58dfa9befa7ee2896ea06d
Reviewed-on: https://cl.tvl.fyi/c/depot/+/1217
Tested-by: BuildkiteCI
Reviewed-by: isomer <isomer@tvl.fyi>

third_party/nix/src/libexpr/CMakeLists.txt

@@ -29,6 +29,7 @@ set(HEADER_FILES
     json-to-value.hh
     names.hh
     nixexpr.hh
+    parser.hh
     primops.hh
     symbol-table.hh
     value.hh