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/decimal.wren

@@ -0,0 +1,8 @@
+IO.print(123.decimal)      // expect: 0
+IO.print((-123).decimal)   // expect: 0
+IO.print(0.decimal)        // expect: 0
+IO.print((-0).decimal)     // expect: -0
+IO.print(0.123.decimal)    // expect: 0.123
+IO.print(12.3.decimal)     // expect: 0.3
+IO.print((-0.123).decimal) // expect: -0.123
+IO.print((-12.3).decimal)  // expect: -0.3