Add WDL language support (#3858)

* Add WDL language support

* Add ace mode

samples/wdl/ifs_in_scatters.wdl

@@ -0,0 +1,44 @@
+# Sample originally from https://github.com/broadinstitute/centaur
+
+task validate_int {
+  Int i
+  command {
+    echo $(( ${i} % 2 ))
+  }
+  output {
+    Boolean validation = read_int(stdout()) == 1
+  }
+  runtime {
+    docker: "ubuntu:latest"
+  }
+}
+
+task mirror {
+  Int i
+  command {
+    echo ${i}
+  }
+  output {
+    Int out = read_int(stdout())
+  }
+  runtime {
+    docker: "ubuntu:latest"
+  }
+}
+
+workflow ifs_in_scatters {
+  Array[Int] numbers = range(5)
+
+  scatter (n in numbers) {
+
+    call validate_int { input: i = n }
+    if (validate_int.validation) {
+      Int incremented = n + 1
+      call mirror { input: i = incremented }
+    }
+  }
+
+  output {
+    Array[Int?] mirrors = mirror.out
+  }
+}