AK+LibRegex: Add Utf16View::code_point_at and use it in RegexStringView

The current method of iterating through the string to access a code
point hurts performance quite badly for very large strings. The test262
test "RegExp/property-escapes/generated/Any.js" previously took 3 hours
to complete; this one change brings it down to under 10 seconds.

AK/Utf16View.cpp

@@ -111,6 +111,23 @@ u16 Utf16View::code_unit_at(size_t index) const
     return m_code_units[index];
 }
 
+u32 Utf16View::code_point_at(size_t index) const
+{
+    VERIFY(index < length_in_code_units());
+
+    u32 code_point = code_unit_at(index);
+    if (!is_high_surrogate(code_point) && !is_low_surrogate(code_point))
+        return code_point;
+    if (is_low_surrogate(code_point) || (index + 1 == length_in_code_units()))
+        return code_point;
+
+    auto second = code_unit_at(index + 1);
+    if (!is_low_surrogate(second))
+        return code_point;
+
+    return decode_surrogate_pair(code_point, second);
+}
+
 size_t Utf16View::code_point_offset_of(size_t code_unit_offset) const
 {
     size_t code_point_offset = 0;

AK/Utf16View.h

@@ -87,6 +87,7 @@ public:
 
     u16 const* data() const { return m_code_units.data(); }
     u16 code_unit_at(size_t index) const;
+    u32 code_point_at(size_t index) const;
 
     size_t code_point_offset_of(size_t code_unit_offset) const;
     size_t code_unit_offset_of(size_t code_point_offset) const;

Userland/Libraries/LibRegex/RegexMatch.h

@@ -240,7 +240,10 @@ public:
                 return ch;
             },
             [&](Utf32View& view) -> u32 { return view[index]; },
+            [&](Utf16View& view) -> u32 { return view.code_point_at(index); },
             [&](auto& view) -> u32 {
+                // FIXME: Iterating to the code point is inefficient, particularly for very large
+                // strings. Implement something like code_point_at to Utf8View.
                 size_t i = index;
                 for (auto it = view.begin(); it != view.end(); ++it, --i) {
                     if (i == 0)