feat: add block scope support with push/pop macros and scope tracking in compiler

Introduce Scope struct and PUSH_SCOPE/POP_SCOPE macros to manage nested variable scopes during compilation. Update declareVariable to distinguish global declarations from local ones based on scope presence rather than parent compiler. Add truncateSymbolTable helper to clean up local symbols when exiting a scope. Include new test files verifying scoped variable isolation in if, while, and block constructs.

test/if.wren

@@ -36,3 +36,11 @@ if
 // Newline after "else".
 if (false) io.write("bad") else
 io.write("good") // expect: good
+
+// Definition in then arm.
+if (true) var a = io.write("ok") // expect: ok
+if (true) class Foo {} // no error
+
+// Definition in else arm.
+if (false) null else var a = io.write("ok") // expect: ok
+if (true) null else class Foo {} // no error

test/scope_if.wren

@@ -0,0 +1,9 @@
+// Create a local scope for the 'then' expression.
+var a = "out"
+if (true) var a = "in"
+io.write(a) // expect: out
+
+// Create a local scope for the 'else' expression.
+var b = "out"
+if (false) "dummy" else var b = "in"
+io.write(b) // expect: out

test/scope_reuse_in_different_blocks.wren

@@ -0,0 +1,9 @@
+{
+  var a = "first"
+  io.write(a) // expect: first
+}
+
+{
+  var a = "second"
+  io.write(a) // expect: second
+}

test/scope_while.wren

@@ -0,0 +1,7 @@
+// Body has its own scope.
+var a = "outer"
+var i = 0
+while ((i = i + 1) <= 1) var a = "inner"
+io.write(a) // expect: outer
+
+// TODO(bob): What about condition?

test/while.wren

@@ -29,3 +29,7 @@ var f = while (e < 3) {
   e = e + 1
 }
 io.write(f) // expect: null
+
+// Definition body.
+while (false) var a = "ok" // no error
+while (false) class Foo {} // no error