feat: add Num.largest and Num.smallest static properties for float limits

Add two new static properties to the Num class: `Num.largest` returns `DBL_MAX`
and `Num.smallest` returns `DBL_EPSILON`. Include corresponding documentation
in the core module markdown and register the primitives in the VM core
initialization.

doc/site/modules/core/num.markdown

@@ -13,6 +13,14 @@ It is a runtime error if `value` is not a string.
 
 The value of π.
 
+### Num.**largest**
+
+The highest representable value of `Num`.
+
+### Num.**smallest**
+
+The difference between 1.0 and the next representable value for `Num`.
+
 ## Methods
 
 ### **abs**