LibJS: Update NumberPrototype's this_number_value() to take a Value

This is now about as close to the spec as it gets - instead of querying
the |this| value inside of the function, we now pass it in from the
outside.
Also get rid of the oddly specific error messages, they're nice but
pretty inconsistent with most others. Let's prefer consistency and
simplicity for now.

Other than that, no functionality change.

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.toString.js

@@ -77,7 +77,7 @@ test("errors", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
             expect(() => Number.prototype.toString.call(value)).toThrowWithMessage(
                 TypeError,
-                "Number.prototype.toString() called with incompatible this target"
+                "Not a Number object"
             );
         });
     });

Userland/Libraries/LibJS/Tests/builtins/Number/Number.prototype.valueOf.js

@@ -14,7 +14,7 @@ test("errors", () => {
         [true, [], {}, Symbol("foo"), "bar", 1n].forEach(value => {
             expect(() => Number.prototype.valueOf.call(value)).toThrowWithMessage(
                 TypeError,
-                "Number.prototype.valueOf() called with incompatible this target"
+                "Not a Number object"
             );
         });
     });