Add swift 5.3

Author: fwal

__tests__/swift-versions.test.ts

@@ -23,7 +23,7 @@ describe("swift version resolver", () => {
 
   it("identifies X versions", async () => {
     const version = await versions.verify("5");
-    expect(version).toBe("5.2.4");
+    expect(version).toBe("5.3");
   });
 
   it("throws an error if version is invalid", async () => {

src/swift-versions.ts

@@ -3,6 +3,7 @@ import * as core from "@actions/core";
 import { System, OS } from "./os";
 
 const AVAILABLE_VERSIONS = [
+  "5.3",
   "5.2.4",
   "5.2.2",
   "5.2.1",