feat: allow empty argument list methods in calls, definitions, and docs

- Compile empty `()` as a valid method signature distinct from no parentheses
- Update `call()`, `clear()`, `run()`, `try()`, and `yield()` to use empty argument lists
- Revise documentation across multiple files to reflect new signature semantics

test/method/arity.wren

@@ -1,5 +1,6 @@
 class Foo {
-  method { 0 }
+  method { "getter" }
+  method() { "no args" }
   method(a) { a }
   method(a, b) { a + b }
   method(a, b, c) { a + b + c }
@@ -19,7 +20,8 @@ class Foo {
 }
 
 var foo = new Foo
-IO.print(foo.method) // expect: 0
+IO.print(foo.method) // expect: getter
+IO.print(foo.method()) // expect: no args
 IO.print(foo.method(1)) // expect: 1
 IO.print(foo.method(1, 2)) // expect: 3
 IO.print(foo.method(1, 2, 3)) // expect: 6