Feature: tier XI compatibility

Prepares BlitzKit for tier XI tanks ahead of their release. Tier X was hardcoded as the maximum in the tank search filters, the guesser and the tech tree, and had no roman numeral beyond X.

- Added `XI` to `TIER_ROMAN_NUMERALS`
- Added an awaitable tier list derived from the highest tier in the tank definitions, so nothing changes until tier XI tanks exist
- Used it for the tank search tier filters and the guesser tier picker and defaults
- Made the tech tree check for successors instead of assuming tier X is the last tier

Fixes #302

packages/website/src/core/awaitables/tiers.ts

@@ -0,0 +1,10 @@
+import { times } from "lodash-es";
+import { api } from "../blitzkit/api";
+
+export const awaitableTiers = api.tankDefinitions().then((tankDefinitions) => {
+  const maxTier = Math.max(
+    ...Object.values(tankDefinitions.tanks).map((tank) => tank.tier),
+  );
+
+  return times(maxTier, (index) => maxTier - index);
+});