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.cpp

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2023, Dan Klishch <danilklishch@gmail.com>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/StringBase.h>
+#include <AK/StringInternals.h>
+
+namespace AK::Detail {
+
+ReadonlyBytes ShortString::bytes() const
+{
+    return { storage, byte_count() };
+}
+
+size_t ShortString::byte_count() const
+{
+    return byte_count_and_short_string_flag >> 1;
+}
+
+StringBase::StringBase(NonnullRefPtr<Detail::StringData const> data)
+    : m_data(&data.leak_ref())
+{
+}
+
+StringBase::StringBase(StringBase const& other)
+    : m_data(other.m_data)
+{
+    if (!is_short_string())
+        m_data->ref();
+}
+
+StringBase& StringBase::operator=(StringBase&& other)
+{
+    if (!is_short_string())
+        m_data->unref();
+
+    m_data = exchange(other.m_data, nullptr);
+    other.m_short_string.byte_count_and_short_string_flag = SHORT_STRING_FLAG;
+    return *this;
+}
+
+StringBase& StringBase::operator=(StringBase const& other)
+{
+    if (&other != this) {
+        if (!is_short_string())
+            m_data->unref();
+
+        m_data = other.m_data;
+        if (!is_short_string())
+            m_data->ref();
+    }
+    return *this;
+}
+
+bool StringBase::is_short_string() const
+{
+    return has_short_string_bit(reinterpret_cast<uintptr_t>(m_data));
+}
+
+}