feat: implement floating-point number literal parsing and arithmetic operations

Add support for floating-point number literals in the parser by detecting decimal points with trailing digits in `readNumber()`, and switch the compiler's `number()` function from `strtol` to `strtod` for proper double conversion. Update the `num_abs` primitive to use `fabs()` instead of manual negation. Extend test coverage across arithmetic, comparison, equality, modulus, and string conversion tests with floating-point cases, and add a new test for decimal point at end-of-file error detection.

test/number_literals.wren

@@ -1,6 +1,10 @@
-io.write(123)    // expect: 123
-io.write(987654) // expect: 987654
-io.write(0)      // expect: 0
-io.write(-0)     // expect: 0
+io.write(123)     // expect: 123
+io.write(987654)  // expect: 987654
+io.write(0)       // expect: 0
+io.write(-0)      // expect: -0
 
-// TODO(bob): Floating point numbers.
+io.write(123.456) // expect: 123.456
+io.write(-0.001)  // expect: -0.001
+
+// TODO(bob): Hex? Scientific notation?
+// TODO(bob): Literals at and beyond numeric limits.