add types

[goto-bus-stop]

Adds typescript types for the secure-json-parse functions.

The any return types are not ideal (it could be better to return unknown and force an explicit cast) but this way it's the same as JSON.parse().

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[mcollina]

Could you add a test test for the types using tsd?

[goto-bus-stop]

yep--turns out i made a mistake in the declarations the first time round! should be accurate now.

[goto-bus-stop]

looks like tsd doesn't support <10.x (even the versions that claim to support 8.x pull in dependencies that no longer do).
I don't know if there's precedent in the fastify org for this, I could add an npm run test-legacy for that doesn't run tsd so node 6 and 8 can still be tested on CI. does that sound good?

[mcollina]

good for me!

[L2jLiga]

LGTM

[mcollina]

lgtm