Support readonly as Flow variance annotation (#19022)

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>
Co-authored-by: fisker Cheung <lionkay@gmail.com>

changelog_unreleased/flow/19022.md

@@ -0,0 +1,19 @@
+#### Support `readonly` as a variance annotation (#19022 by @marcoww6)
+
+Flow now accepts `readonly` as a property variance annotation, equivalent to `+` (covariant/read-only).
+
+<!-- prettier-ignore -->
+```jsx
+// Input
+type T = {
+  readonly foo: string,
+};
+
+// Prettier stable
+SyntaxError
+
+// Prettier main
+type T = {
+  readonly foo: string,
+};
+```