Bugfix: Printing of native class prototypes

[ryutamago]

Context

There was a bug associated with converting the prototype of URL and URLSearchParam to string.
This is because the to_string methods we have only handles the conversion fromURL and URLSearchParam, not the their prototypes.

Description

Made changes to the URL and URLSearchParam so that in case the js value is not of type Self, call the builtin Object's to_string method

Manually testing the PR

In repl,

>> console.log(URL)
[🪵] function URL() { [native code] }
>> console.log(URLSearchParams)
[🪵] function URLSearchParams() { [native code] }
>> console.log(URL.prototype)
[🪵] [object Object]
>> console.log(URLSearchParams.prototype)
[🪵] [object Object]

[johnyob]

LGTM.

Please squash before merging (commit history is a little messy)