feat: add Num.fromString static method to parse decimal string literals

Implements a new `fromString` static method on the `Num` class that attempts to parse a string as a decimal number literal using `strtod`. Returns the parsed `Num` value on success, or `null` if the string does not represent a valid number. Includes runtime validation that the argument is a string, and adds test coverage for positive, negative, zero, and decimal inputs as well as non-numeric strings.

doc/site/core/num.markdown

@@ -90,3 +90,10 @@ It is a runtime error if `other` is not a number.
 ### **...**(other) operator
 
 **TODO**
+
+### Num.**fromString**(value)
+
+Attempts to parse `value` as a decimal literal and return it as an instance of
+`Num`. If the number cannot be parsed `null` will be returned.
+
+It is a runtime error if `value` is not a string.