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/yield.wren

@@ -1,14 +1,14 @@
 var fiber = new Fiber {
   IO.print("fiber 1")
-  Fiber.yield
+  Fiber.yield()
   IO.print("fiber 2")
-  Fiber.yield
+  Fiber.yield()
   IO.print("fiber 3")
 }
 
-var result = fiber.call // expect: fiber 1
-IO.print("main 1")  // expect: main 1
-result = fiber.call // expect: fiber 2
-IO.print("main 2")  // expect: main 2
-result = fiber.call // expect: fiber 3
-IO.print("main 3")  // expect: main 3
+var result = fiber.call() // expect: fiber 1
+IO.print("main 1")    // expect: main 1
+result = fiber.call() // expect: fiber 2
+IO.print("main 2")    // expect: main 2
+result = fiber.call() // expect: fiber 3
+IO.print("main 3")    // expect: main 3