[ox/std] Add non-const operator[] to Pair

(synced from 3824788a1a)

src/ox/std/span.hpp

@@ -228,6 +228,10 @@ class Span {
 			return iterator<const T&, const T*, true>(m_items, MaxValue<size_type>, m_size);
 		}
 
+		constexpr T &operator[](std::size_t i) noexcept {
+			return m_items[i];
+		}
+
 		constexpr const T &operator[](std::size_t i) const noexcept {
 			return m_items[i];
 		}