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/break/closure_in_for.wren

@@ -5,5 +5,5 @@ for (i in [1, 2, 3]) {
   break
 }
 
-f.call
+f.call()
 // expect: 5

test/break/closure_in_while.wren

@@ -5,5 +5,5 @@ while (true) {
   break
 }
 
-f.call
+f.call()
 // expect: i