feat: sealed and readonly utilities

Drop support for TypeScript 3.x

It may still work, but we no longer test on TypeScript < 4
Drop support for node.js 10

It may still work, but we no longer test on node.js < 12
Add "exports" to package.json

This is very unlikely to break your code, but it could cause issues if you were relying on importing a custom path from within this package. This update does also mean that (in environments that support it) the ESModules version of this library will be used in place of the CommonJS version

Added 'funtypes/readonly' version of the package that flips the defaults

Usage:
```
import * as t from 'funtypes/readonly';
```
This replaces Object with ReadonlyObject, Array with ReadonlyArray etc.
Added MutableObject, MutableArray etc. to allow explicitly mutable versions to be used when using the new 'funtypes/readonly' export
Added Mutable utility to do the inverse of the Readonly utility

Added Sealed utility to trigger an error for unexpected additional properties on objects

Usage:

import * as t from 'funtypes';

const MySealedSchema = Sealed(MySchema);

const MyDeepSealedSchema = Sealed(MySchema, {deep: true})

ForbesLindesay / funtypes