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_divide.wren

@@ -1,5 +1,5 @@
-io.write(8 / 2) // expect: 4
+io.write(8 / 2)        // expect: 4
+io.write(12.34 / -0.4) // expect: -30.85
 
-// TODO(bob): Floating point numbers.
 // TODO(bob): Unsupported RHS types.
 // TODO(bob): Divide by zero.