test: add test files for decimal, div, sign, and truncate methods on Number

Add four new test files covering edge cases for Number methods:
- decimal.wren tests fractional part extraction including negative zero
- div.wren tests integer division behavior with mixed integer/float operands
- sign.wren tests sign determination including zero and negative zero
- truncate.wren tests truncation of positive and negative numbers

test/number/sign.wren

@@ -0,0 +1,6 @@
+IO.print(123.sign)      // expect: 1
+IO.print((-123).sign)   // expect: -1
+IO.print(0.sign)        // expect: 0
+IO.print((-0).sign)     // expect: 0
+IO.print(0.123.sign)    // expect: 1
+IO.print((-0.123).sign) // expect: -1