Fix defaultFilter code injection

[bgub]

Summary

• serialize defaultFilter with JSON.stringify before embedding it in generated function source
• update the compile-string expectation for the generated filter lookup
• add a regression test that malicious defaultFilter text is treated as a filter name and does not execute

Testing

• npx jest test/compile-string.spec.ts test/filters.spec.ts --runInBand --no-cache
• npm test -- --runInBand --no-cache
• npm run lint (passes with existing warnings)

Notes

• npm run build currently gets through Rollup, then fails in TypeDoc while parsing node_modules/@types/babel__traverse/index.d.ts with the repo's pinned TypeScript 4.0.8.