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.

test/number/from_string.wren

@@ -0,0 +1,8 @@
+IO.print(Num.fromString("123") == 123) // expect: true
+IO.print(Num.fromString("-123") == -123) // expect: true
+IO.print(Num.fromString("-0") == -0) // expect: true
+IO.print(Num.fromString("12.34") == 12.34) // expect: true
+IO.print(Num.fromString("-0.0001") == -0.0001) // expect: true
+
+// Test a non-number literal and ensure it returns null.
+IO.print(Num.fromString("test1") == null) // expect: true

test/number/literals.wren

@@ -6,5 +6,5 @@ IO.print(-0)      // expect: -0
 IO.print(123.456) // expect: 123.456
 IO.print(-0.001)  // expect: -0.001
 
-// TODO: Hex? Scientific notation?
+// TODO: Scientific notation?
 // TODO: Literals at and beyond numeric limits.