AK: Make String::number() infallible

This API will always succeed in creating a String representing the
provided number in base-10.

Userland/Libraries/LibWeb/CSS/StyleValues/NumberStyleValue.cpp

@@ -13,7 +13,7 @@ namespace Web::CSS {
 
 String NumberStyleValue::to_string() const
 {
-    return MUST(String::number(m_value));
+    return String::number(m_value);
 }
 
 }