LibJS: Cache the number format used for compact notation

Finding the best number format to use for compact notation involves
creating a Vector of all compact formats for the locale and looking for
the one that best matches the number's magnitude. ECMA-402 wants this
number format to be found multiple times, so cache the result for future
use.

Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.cpp

@@ -11,7 +11,6 @@
 #include <LibJS/Runtime/Intl/NumberFormat.h>
 #include <LibJS/Runtime/Intl/NumberFormatFunction.h>
 #include <LibUnicode/CurrencyCode.h>
-#include <LibUnicode/Locale.h>
 #include <math.h>
 #include <stdlib.h>
 
@@ -1599,7 +1598,11 @@ int compute_exponent_for_magniude(NumberFormat& number_format, int magnitude)
             best_number_format = &format_rule;
         }
 
-        return best_number_format ? best_number_format->exponent : 0;
+        if (best_number_format == nullptr)
+            return 0;
+
+        number_format.set_compact_format(*best_number_format);
+        return best_number_format->exponent;
     }
 
     default:

Userland/Libraries/LibJS/Runtime/Intl/NumberFormat.h

@@ -10,6 +10,7 @@
 #include <AK/String.h>
 #include <LibJS/Runtime/Intl/AbstractOperations.h>
 #include <LibJS/Runtime/Object.h>
+#include <LibUnicode/Locale.h>
 
 namespace JS::Intl {
 
@@ -155,6 +156,10 @@ public:
     NativeFunction* bound_format() const { return m_bound_format; }
     void set_bound_format(NativeFunction* bound_format) { m_bound_format = bound_format; }
 
+    bool has_compact_format() const { return m_compact_format.has_value(); }
+    void set_compact_format(Unicode::NumberFormat compact_format) { m_compact_format = compact_format; }
+    Unicode::NumberFormat compact_format() const { return *m_compact_format; }
+
 private:
     virtual void visit_edges(Visitor&) override;
 
@@ -181,6 +186,9 @@ private:
 
     // Non-standard. Stores the resolved currency display string based on [[Locale]], [[Currency]], and [[CurrencyDisplay]].
     Optional<StringView> m_resolved_currency_display;
+
+    // Non-standard. Stores the resolved compact number format based on [[Locale]], [[Notation], [[Style]], and [[CompactDisplay]].
+    Optional<Unicode::NumberFormat> m_compact_format;
 };
 
 struct FormatResult {