LibJS: Manually loop over escaped regex pattern instead of ::replace()

This makes it ever-so-slightly faster, but more importantly, it fixes
the bug where a `/\//` regex's `source` property would return `\\/`
("\\\\/") instead of `\/` due to the existing '/' -> '\/' replace()
call.

Userland/Libraries/LibJS/Tests/builtins/RegExp/RegExp.prototype.source.js

@@ -3,6 +3,5 @@ test("basic functionality", () => {
     expect(RegExp().source).toBe("(?:)");
     expect(/test/.source).toBe("test");
     expect(/\n/.source).toBe("\\n");
-    // FIXME: RegExp parse doesn't parse \/ :(
-    // expect(/foo\/bar/.source).toBe("foo\\/bar");
+    expect(/foo\/bar/.source).toBe("foo\\/bar");
 });