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/limit/long_function.wren

@@ -1009,5 +1009,5 @@ new Fn {
   IO.print(9) // expect: 9
   IO.print(10) // expect: 10
   // 1000
-}.call
+}.call()
 

test/limit/many_constants.wren

@@ -65537,4 +65537,4 @@ var f = new Fn {
   IO.print(65536)
 }
 
-f.call // expect: 65536
+f.call() // expect: 65536