test: add scope edge case tests for variable initialization and shadowing

Add four new test files covering variable scope edge cases:
- global_in_initializer: verify global variable can reference itself in initializer
- local_in_initializer: verify local variable referencing itself in initializer produces error
- shadow_and_local: verify shadowed variable correctly resolves to outer scope before inner declaration
- shadow_in_initializer: verify shadowed variable in initializer correctly references outer scope value

test/variable/shadow_and_local.wren

@@ -0,0 +1,8 @@
+{
+  var a = "outer"
+  {
+    io.write(a) // expect: outer
+    var a = "inner"
+    io.write(a) // expect: inner
+  }
+}