AK: Add a Utf8View type for iterating over UTF-8 codepoints

Utf8View wraps a StringView and implements begin() and end() that
return a Utf8CodepointIterator, which parses UTF-8-encoded Unicode
codepoints and returns them as 32-bit integers.

This is the first step towards supporting emojis in Serenity ^)
https://github.com/SerenityOS/serenity/issues/490

AK/Utf8View.h

@@ -0,0 +1,48 @@
+#pragma once
+
+#include <AK/StringView.h>
+#include <AK/Types.h>
+
+namespace AK {
+
+class Utf8View;
+
+class Utf8CodepointIterator {
+    friend class Utf8View;
+
+public:
+    ~Utf8CodepointIterator() {}
+
+    bool operator==(const Utf8CodepointIterator&) const;
+    bool operator!=(const Utf8CodepointIterator&) const;
+    Utf8CodepointIterator& operator++();
+    u32 operator*() const;
+
+private:
+    Utf8CodepointIterator(const unsigned char*, int);
+    const unsigned char* m_ptr { nullptr };
+    int m_length { -1 };
+};
+
+class Utf8View {
+public:
+    explicit Utf8View(const StringView&);
+    ~Utf8View() {}
+
+    const StringView& as_string() const { return m_string; }
+
+    Utf8CodepointIterator begin() const;
+    Utf8CodepointIterator end() const;
+
+    bool validate() const;
+
+private:
+    const unsigned char* begin_ptr() const;
+    const unsigned char* end_ptr() const;
+
+    StringView m_string;
+};
+
+}
+
+using AK::Utf8View;