feat: allow readonly arrays in JSON schema

[cyrilletuzi]

Given how JSON schema works, it is better to set them as const:

this.storage.get('name', { type: 'string' } as const);

But before v11, it was not possible when the JSON schema was using properties of array type (enum, items, required). This is now fixed, and is a first step toward auto-inferring the type from the JSON schema in all scenarios (#463).

Fixes #470