feat(ts/models/shuttle): create RunId to ShuttleVariant "mapper"

The `RunId` => `ShuttleVariant` relationship is "many to one", which makes this better suited for a function which returns the enum value from the `RunId`.

assets/src/models/shuttle.ts

@@ -1,4 +1,4 @@
-import { Vehicle } from "../realtime"
+import { RunId, Vehicle } from "../realtime"
 
 export enum ShuttleVariant {
   // Rapid Transit Lines
@@ -12,6 +12,27 @@ export enum ShuttleVariant {
   Special,
 }
 
+export const shuttleVariantFromRunId = (
+  runId: RunId
+): ShuttleVariant | null => {
+  switch (runId) {
+    case "999-0501":
+      return ShuttleVariant.Blue
+    case "999-0502":
+      return ShuttleVariant.Green
+    case "999-0503":
+      return ShuttleVariant.Orange
+    case "999-0504":
+      return ShuttleVariant.Red
+    case "999-0505":
+      return ShuttleVariant.CommuterRail
+    case "999-0555":
+      return ShuttleVariant.Special
+    default:
+      return null
+  }
+}
+
 export const formattedRunNumber = ({ runId }: Vehicle): string => {
   if (runId === null) {
     return "Not Available"