AK: Move data fields from AK::String to a newly created AK::StringBase

This starts separating memory management of string data and string
utilities like `String::formatted`. This would also allow to reuse the
same storage in `DeprecatedString` in the future.

AK/StringBase.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <AK/Endian.h>
+#include <AK/Forward.h>
+
+namespace AK::Detail {
+
+class StringData;
+
+static constexpr size_t MAX_SHORT_STRING_BYTE_COUNT = sizeof(StringData*) - 1;
+
+struct ShortString {
+    ReadonlyBytes bytes() const;
+    size_t byte_count() const;
+
+    // NOTE: This is the byte count shifted left 1 step and or'ed with a 1 (the SHORT_STRING_FLAG)
+    u8 byte_count_and_short_string_flag { 0 };
+    u8 storage[MAX_SHORT_STRING_BYTE_COUNT] = { 0 };
+};
+
+static_assert(HostIsLittleEndian, "Order of fields in ShortString assumes LE.");
+static_assert(sizeof(ShortString) >= sizeof(StringData*));
+static_assert(__builtin_offsetof(ShortString, byte_count_and_short_string_flag) == 0);
+
+class StringBase {
+public:
+    StringBase(StringBase const&);
+
+    constexpr StringBase(StringBase&& other)
+        : m_short_string(other.m_short_string)
+    {
+        other.m_short_string = ShortString {};
+        other.m_short_string.byte_count_and_short_string_flag = SHORT_STRING_FLAG;
+    }
+
+    StringBase& operator=(StringBase&&);
+    StringBase& operator=(StringBase const&);
+
+    // NOTE: This is primarily interesting to unit tests.
+    [[nodiscard]] bool is_short_string() const;
+
+protected:
+    // NOTE: If the least significant bit of the pointer is set, this is a short string.
+    static constexpr uintptr_t SHORT_STRING_FLAG = 1;
+
+    static constexpr bool has_short_string_bit(uintptr_t data)
+    {
+        return (data & SHORT_STRING_FLAG) != 0;
+    }
+
+    explicit StringBase(NonnullRefPtr<Detail::StringData const>);
+
+    explicit constexpr StringBase(ShortString short_string)
+        : m_short_string(short_string)
+    {
+    }
+
+    union {
+        ShortString m_short_string;
+        Detail::StringData const* m_data { nullptr };
+    };
+};
+
+}