refactor: move benchmark directory under test and update all references

The benchmark directory is relocated from the project root into the test directory, and all scripts, documentation links, and gitignore paths are updated accordingly. The test runner is also refactored to explicitly walk subdirectories (core, io, language, limit) instead of using a single test root, and a new test/README.md is added to document the test suite structure. Additionally, the constructor test for the Foo class is updated to add a zero-arity constructor and adjust expected output strings.

test/language/is/is.wren

@@ -0,0 +1,33 @@
+IO.print(Num is Class) // expect: true
+IO.print(true is Bool) // expect: true
+IO.print(new Fn { 1 } is Fn) // expect: true
+IO.print(123 is Num) // expect: true
+IO.print(null is Null) // expect: true
+IO.print("s" is String) // expect: true
+
+IO.print(Num is Bool) // expect: false
+IO.print(null is Class) // expect: false
+IO.print(true is Fn) // expect: false
+IO.print(new Fn { 1 } is Num) // expect: false
+IO.print("s" is Null) // expect: false
+IO.print(123 is String) // expect: false
+
+// Everything extends Object.
+IO.print(Num is Object) // expect: true
+IO.print(null is Object) // expect: true
+IO.print(true is Object) // expect: true
+IO.print(new Fn { 1 } is Object) // expect: true
+IO.print("s" is Object) // expect: true
+IO.print(123 is Object) // expect: true
+
+// Classes extend Class.
+IO.print(Num is Class) // expect: true
+IO.print(null is Class) // expect: false
+IO.print(true is Class) // expect: false
+IO.print(new Fn { 1 } is Class) // expect: false
+IO.print("s" is Class) // expect: false
+IO.print(123 is Class) // expect: false
+
+// Ignore newline after "is".
+IO.print(123 is
+  Num) // expect: true

test/language/is/nonclass_on_right.wren

@@ -0,0 +1 @@
+1 is false // expect runtime error: Right operand must be a class.