Backslashes are wrongly escaped in grammar_codegen

[racko]

When I change fuzztest/grammars/JSON.g4 to support some escape sequences

STRING : '"' (CHARACTER | '\\' ESCAPED )* '"' ;

ESCAPED : '"' | '\\' | '/' | 'b' | 'f' | 'n' | 'r' | 't' ;

then the grammar tests fail because the nlohmann/json library fails to parse the generated json:

$ bazel test //domain_tests:in_grammar_domain_test
...
Executing tests from //domain_tests:in_grammar_domain_test
-----------------------------------------------------------------------------
Running main() from gmock_main.cc
[==========] Running 6 tests from 2 test suites.
[----------] Global test environment set-up.
[----------] 5 tests from InJsonGrammar
[ RUN      ] InJsonGrammar.InitGeneratesDifferentValidJson
domain_tests/in_grammar_domain_test.cc:41: Failure
Value of: parsed_json.is_discarded()
  Actual: true
Expected: false
"{\"\\\\\"\":[80.0E3,false]}"
...

That's because backslashes are escaped when they shouldn't be. E.g. "\"" (quote, escaped quote, quote) becomes "\\"" (quote, escaped backslash, quote, quote), which is not correct json.

The linked PR fixes the issue.