Update type of string-join for its new keyword arguments.

Closes PR12866.

original commit: b1ad108759d05a85a04f7220bb79e2b70156aaf9

collects/tests/typed-racket/unit-tests/typecheck-tests.rkt

@@ -337,6 +337,9 @@
            (let: ([v : String "a"])
                  (string-append "foo" (a v))))
          -String]
+        [tc-e (string-join '("hello" "world") " ") -String]
+        [tc-e (string-join '("hello" "world")) -String]
+        [tc-e (string-join '("hello" "world") #:first "a") -String]
 
         [tc-e (apply (plambda: (a) [x : a *] x) '(5)) (-lst -PosByte)]
         [tc-e (apply append (list '(1 2 3) '(4 5 6))) (-lst -PosByte)]

collects/typed-racket/base-env/base-env.rkt

@@ -363,7 +363,10 @@
  (cl->* (-> (-lst -String) -String)
         (-> -String (-lst -String) -String))]
 
-[string-join (-> (-lst -String) -String -String)]
+[string-join
+ (->optkey (-lst -String) [-String]
+           #:before-last -String #f #:first -String #f #:last -String #f
+           -String)]
 
 
 ;Section 3.6