AK+Everywhere: Stop including Vector.h from StringView.h

Preparation for using Error.h from Vector.h. This required moving some
things out of line.

AK/String.cpp

@@ -479,4 +479,9 @@ String String::vformatted(StringView fmtstr, TypeErasedFormatParams& params)
     return builder.to_string();
 }
 
+Vector<size_t> String::find_all(StringView needle) const
+{
+    return StringUtils::find_all(*this, needle);
+}
+
 }