Improve long mapped type to across multiple line in Flow (#18779)

Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/flow/18779.md

@@ -0,0 +1,25 @@
+#### Improve long mapped type to across multiple line (#18779 by @kovsu)
+
+<!-- prettier-ignore -->
+```flow
+// Input
+type MappedType = {
+  [KeyType in ValueType extends string ? VeryLongStringTypeNameHere : VeryLongNumberTypeNameHere]: KeyType
+}
+
+// Prettier stable
+type MappedType = {
+  [KeyType in ValueType extends string
+      ? VeryLongStringTypeNameHere
+      : VeryLongNumberTypeNameHere]: KeyType,
+};
+
+// Prettier main
+type MappedType = {
+  [
+    KeyType in ValueType extends string
+      ? VeryLongStringTypeNameHere
+      : VeryLongNumberTypeNameHere
+  ]: KeyType,
+};
+```