Implement a new `Num.fromString(value)` method that attempts to parse a string as a decimal literal and returns the corresponding `Num` instance, or `null` if parsing fails. The implementation uses `strtod` for conversion, validates the argument is a string at runtime, and includes tests for valid numbers, edge cases like leading/trailing whitespace and partial suffix parsing, non-number inputs returning null, and a runtime error when a non-string argument is passed. Also fixes a missing semicolon in the `toString` native registration and removes the hex literal TODO from the number literals test.