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/div.wren

@@ -0,0 +1,5 @@
+IO.print((8).div(2))   // expect: 4
+IO.print((8.5).div(2)) // expect: 4
+IO.print((9).div(2))   // expect: 4
+IO.print((9.9).div(2)) // expect: 4
+IO.print((10).div(2))  // expect: 5