docs: document is operator as overridable infix method and add type-test examples

test/language/method/operators.wren

@@ -14,6 +14,7 @@ class Foo {
   !=(other) { "infix != " + other }
   &(other) { "infix & " + other }
   |(other) { "infix | " + other }
+  is(other) { "infix is " + other }
 
   ! { "prefix !" }
   ~ { "prefix ~" }
@@ -37,3 +38,4 @@ System.print(foo | "a") // expect: infix | a
 System.print(!foo) // expect: prefix !
 System.print(~foo) // expect: prefix ~
 System.print(-foo) // expect: prefix -
+System.print(foo is "a") // expect: infix is a