LibJS: Transcode UTF-8 strings to UTF-16 and add UTF-16 accessors

LibJS parses JavaScript as UTF-8, so when creating a string, we must
transcode it to UTF-16 to handle encoded surrogate pairs.

For example, consider the following string:
    "\ud83d\ude00"

The UTF-8 encoding of this surrogate pair is:
    0xf0 0x9f 0x98 0x80

However, LibJS will currently store the two surrogates individually as
UTF-8 encoded bytes, rather than combining the pair:
    0xed 0xa0 0xb8, 0xed 0xb8 0x80

These are not equivalent. So, as String.prototype becomes UTF-16 aware,
this encoding will no longer work for abstractions like strict equality.

Userland/Libraries/LibJS/Runtime/PrimitiveString.cpp

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
+#include <AK/CharacterTypes.h>
+#include <AK/Utf16View.h>
 #include <LibJS/Runtime/PrimitiveString.h>
 #include <LibJS/Runtime/VM.h>
 
@@ -18,13 +20,59 @@ PrimitiveString::~PrimitiveString()
 {
 }
 
+Vector<u16> const& PrimitiveString::utf16_string() const
+{
+    if (m_utf16_string.is_empty() && !m_string.is_empty())
+        m_utf16_string = AK::utf8_to_utf16(m_string);
+    return m_utf16_string;
+}
+
+Utf16View PrimitiveString::utf16_string_view() const
+{
+    return Utf16View { utf16_string() };
+}
+
+PrimitiveString* js_string(Heap& heap, Utf16View const& string)
+{
+    if (string.is_empty())
+        return &heap.vm().empty_string();
+
+    if (string.length_in_code_units() == 1) {
+        u16 code_unit = string.code_unit_at(0);
+        if (is_ascii(code_unit))
+            return &heap.vm().single_ascii_character_string(static_cast<u8>(code_unit));
+    }
+
+    auto utf8_string = string.to_utf8(Utf16View::AllowInvalidCodeUnits::Yes);
+    return heap.allocate_without_global_object<PrimitiveString>(move(utf8_string));
+}
+
+PrimitiveString* js_string(VM& vm, Utf16View const& string)
+{
+    return js_string(vm.heap(), string);
+}
+
 PrimitiveString* js_string(Heap& heap, String string)
 {
     if (string.is_empty())
         return &heap.vm().empty_string();
 
-    if (string.length() == 1 && (u8)string.characters()[0] < 0x80)
-        return &heap.vm().single_ascii_character_string(string.characters()[0]);
+    if (string.length() == 1) {
+        auto ch = static_cast<u8>(string.characters()[0]);
+        if (is_ascii(ch))
+            return &heap.vm().single_ascii_character_string(ch);
+    }
+
+    // UTF-8 strings must first be transcoded to UTF-16, even though they are stored as String objects
+    // internally, to parse encoded surrogate pairs. As an optimization to reduce string copying, only
+    // perform that transcoding if there are non-ASCII codepoints in the string.
+    for (auto it : string) {
+        auto ch = static_cast<u8>(it);
+        if (!is_ascii(ch)) {
+            auto utf16_string = AK::utf8_to_utf16(string);
+            return js_string(heap, Utf16View { utf16_string });
+        }
+    }
 
     return heap.allocate_without_global_object<PrimitiveString>(move(string));
 }