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/resume_caller.wren

@@ -4,12 +4,12 @@ var b = new Fiber {
 
 var a = new Fiber {
   IO.print("begin fiber a")
-  b.call
+  b.call()
   IO.print("end fiber a")
 }
 
 IO.print("begin main")
-a.call
+a.call()
 IO.print("end main")
 
 // expect: begin main