LibJS: Implement a basic Intl mathematical value

The Intl mathematical value is much like ECMA-262's mathematical value
in that it is meant to represent an arbitrarily precise number. The Intl
MV further allows positive/negative infinity, negative zero, and NaN.

This implementation is *not* arbitrarily precise. Rather, it is a
replacement for the use of Value within Intl.NumberFormat. The exact
syntax of the Intl MV is still being worked on, but abstracting this
away into its own class will allow hooking in the finalized Intl MV
more easily, and makes implementing Intl.NumberFormat.formatRange
easier.

Note the methods added here are essentially the same as the static
helpers in Intl/NumberFormat.cpp.

Userland/Libraries/LibJS/Forward.h

@@ -225,6 +225,8 @@ namespace Intl {
 JS_ENUMERATE_INTL_OBJECTS
 #undef __JS_ENUMERATE
 
+class MathematicalValue;
+
 // Not included in JS_ENUMERATE_INTL_OBJECTS due to missing distinct constructor
 class Segments;
 class SegmentsPrototype;