AK: Add String::number() for creating a String from a number.

Instead of manually doing String::format("%d"/"%u") everywhere, let's have
a String API for this. It's just a wrapper around format() for now, but it
could be made more efficient in the future.

AK/String.cpp

@@ -170,6 +170,16 @@ unsigned String::to_uint(bool& ok) const
     return value;
 }
 
+String String::number(unsigned value)
+{
+    return String::format("%u", value);
+}
+
+String String::number(int value)
+{
+    return String::format("%d", value);
+}
+
 String String::format(const char* fmt, ...)
 {
     StringBuilder builder;