fix: accommodate for optional properties in index signature

[olistic]

Description

This is an alternative to #86, which fixes the index signatures not supporting optional (undefined) values.

#86 modifies the Json type adding undefined to it, which may be considered not technically correct. This PR proposes an alternative way that doesn't touch the Json type.

Type of Change

• Bug fix

Does this introduce a breaking change?

No.

Are documentation updates required?

No.

Testing information

Build the project and verify the TS compiler doesn't show errors in decode.d.ts for optional properties. Example:

Property 'alg' of type 'string | undefined' is not assignable to 'string' index type 'Json'.

Checklist

• [x] Code is commented where needed
• [x] Unit test coverage for new or modified code paths
• [x] npm run test passes
• [ ] Changelog is updated
• [ ] Tag 1 of @person1 or @person2

[CLAassistant]

All committers have signed the CLA.

[olistic]

@janniks anything else I can do to help landing this?

[janniks]

I think it looks good 👍🏻 Thanks for the ping. I'll add a second reviewer as well.