feat(cache): replace LruCache and TtlCache with unified Cache

Introduces a single `Cache` class that subsumes both `LruCache` and
`TtlCache` into a configuration-driven API (maxSize, ttl, sliding
expiration, stale-while-revalidate refresh, and onRemove). The new
implementation delegates expiration tracking to `IndexedHeap` from
`@std/data-structures/unstable-indexed-heap` for O(log n) evictions.

- Add cache/cache.ts and cache/cache_test.ts
- Remove cache/lru_cache.ts, cache/ttl_cache.ts and their tests
- Update cache/memoize.ts doc to reference Cache
- Update cache/mod.ts and cache/deno.json exports

Made-with: Cursor

Author: tomas-zijdemans

cache/cache.ts

@@ -3,8 +3,7 @@
   "version": "0.2.3",
   "exports": {
     ".": "./mod.ts",
-    "./lru-cache": "./lru_cache.ts",
-    "./memoize": "./memoize.ts",
-    "./ttl-cache": "./ttl_cache.ts"
+    "./cache": "./cache.ts",
+    "./memoize": "./memoize.ts"
   }
 }