Add test cases for Num.fromString covering leading/trailing whitespace (" 12 "),
numeric prefix with trailing non-numeric characters ("1.2prefix"), empty string,
non-numeric prefix ("prefix1.2"), and a runtime error test for non-string argument
(Num.fromString(1)).
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.
