feat: implement ! operator for all core types with proper truthiness semantics

Add native `!` methods to Null, Object, Num, String, and List classes so that `!` works reliably on any value. Null returns `true`, all other types return `false` since they are considered truthy. Update documentation to describe the operator as returning the logical complement and add comprehensive tests for each type. Also fix the `all` method test to use a truthy string instead of expecting a runtime error.

test/null/literal.wren

@@ -0,0 +1 @@
+IO.print(null) // expect: null

test/null/not.wren

@@ -0,0 +1 @@
+IO.print(!null) // expect: true

test/null/type.wren

@@ -0,0 +1,4 @@
+IO.print(null is Null)      // expect: true
+IO.print(null is Object)    // expect: true
+IO.print(null is Bool)      // expect: false
+IO.print(null.type == Null) // expect: true