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/fiber/run_when_done.wren

@@ -5,9 +5,9 @@ var a = new Fiber {
 // Run a through an intermediate fiber since it will get discarded and we need
 // to return to the main one after a completes.
 var b = new Fiber {
-  a.run
+  a.run()
   IO.print("nope")
 }
 
-b.call // expect: run
-a.run // expect runtime error: Cannot run a finished fiber.
+b.call() // expect: run
+a.run()  // expect runtime error: Cannot run a finished fiber.