Stricter DeepReadonly (DeepImmutable?)

[danielnixon]

The DeepReadonly from ts-essentials is great but has some limitations that we could solve with a stricter version of it:

• It leaves built-in types like Date alone - we could replace these with our ReadonlyDate.
• It returns ReadonlySet and ReadonlyMap types - we know these aren't truly readonly and could replace them with our ImmutableSet and ImmutableMap.
• It doesn't replace URLSearchParams, URL - we have readonly versions of these to replace them with.
• Once we've done those three, other opportunities will likely present themselves

[danielnixon]

Coming together: https://github.com/agiledigital/readonly-types/blob/master/src/index.ts#L8-L48

[danielnixon]

:shipit: