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_with_no_caller.wren

@@ -1,9 +1,9 @@
 var a = new Fiber {
-  Fiber.yield // expect runtime error: No fiber to yield to.
+  Fiber.yield() // expect runtime error: No fiber to yield to.
 }
 
 // Run a chain of fibers. Since none of them are called, they all get discarded
 // and there is no remaining caller.
-var b = new Fiber { a.run }
-var c = new Fiber { b.run }
-c.run
+var b = new Fiber { a.run() }
+var c = new Fiber { b.run() }
+c.run()