Add support for "nonlocal" (capitalized) names, and change how variable lookup works.

Inside a method, all local variable lookup stops at the method boundary. In other
words, methods, do not close over outer local variables.

If a name is not found inside a method and is lowercase, it's a method on this.
If it's capitalized, it's a global variable.

test/closure/close_over_method_parameter.wren

@@ -1,12 +1,12 @@
-var f = null
+var F = null
 
 class Foo {
   method(param) {
-    f = new Fn {
+    F = new Fn {
       IO.print(param)
     }
   }
 }
 
 (new Foo).method("param")
-f.call // expect: param
+F.call // expect: param

test/nonlocal/assignment.wren

@@ -0,0 +1,18 @@
+var Nonlocal = "before"
+IO.print(Nonlocal) // expect: before
+Nonlocal = "after"
+IO.print(Nonlocal) // expect: after
+
+class Foo {
+  static method {
+    Nonlocal = "method"
+  }
+}
+
+Foo.method
+IO.print(Nonlocal) // expect: method
+
+new Fn {
+  Nonlocal = "fn"
+}.call
+IO.print(Nonlocal) // expect: fn

test/nonlocal/duplicate_nonlocal.wren

@@ -0,0 +1,6 @@
+class Foo {
+  bar {
+    var A = "value"
+    var A = "other" // expect error
+  }
+}

test/nonlocal/in_block_scope.wren

@@ -0,0 +1,8 @@
+var Nonlocal = "outer"
+
+{
+  var Nonlocal = "inner"
+  IO.print(Nonlocal) // expect: inner
+}
+
+IO.print(Nonlocal) // expect: outer

test/nonlocal/use_global_in_function.wren

@@ -1,6 +1,6 @@
-var global = "global"
+var Global = "global"
 // TODO: Forward reference to global declared after use.
 
 new Fn {
-  IO.print(global) // expect: global
+  IO.print(Global) // expect: global
 }.call

test/nonlocal/use_global_in_method.wren

@@ -1,13 +1,13 @@
-var global = "global"
+var Global = "global"
 // TODO: Forward reference to global declared after use.
 
 class Foo {
   method {
-    IO.print(global)
+    IO.print(Global)
   }
 
   static classMethod {
-    IO.print(global)
+    IO.print(Global)
   }
 }
 

test/variable/outside_method.wren

@@ -0,0 +1,18 @@
+var foo = "variable"
+
+class Foo {
+  foo { "method" }
+
+  method {
+    IO.print(foo)
+  }
+
+  static foo { "class method" }
+
+  static classMethod {
+    IO.print(foo)
+  }
+}
+
+(new Foo).method // expect: method
+Foo.classMethod  // expect: class method