add vitest unit tests

[clockelliptic]

To Do: add vitest unit test cases

Acceptance criteria:

Implementation: Unit tests implemented with vitest.

Test Cases:

Asset-handling behavior:

For both link (CSS stylesheets) and img tags, validates the following

• caches assets as expected
• caches lazy-loaded assets
  • for img assets, checks for lazy="true"
  • for link tags, see here: https://web.dev/articles/defer-non-critical-css
• gracefully handles missing asset caches
• gracefully handles invalid or corrupted asset caches
• respects custom asset cache keys on asset elements (data-key)
• (img tags only) detects browsers without IndexedDB Blob Storage support and caches images as ArrayBuffers

Instance constructor opts:

• caches and loads tag types included in IndexedCacheOpts["tags"], no more no less
  • gracefully handles invalid tag names
• IndexedCacheOpts["skip"] works as intended, forcing assets to be fetched without caching
• IndexedCacheOpts["prune"] removes all cache entries that are not found on the given page
• IndexedCacheOpts["expiry"] cache expiry works as intended

Instance methods:

• instance method IDBSideloader.load([...els]) selectively loads one or more cached assets
  • gracefully handles invalid elements
• instance method IDBSideloader.prune([...keys]) prunes all objects in the given DB except the provided keys
  • gracefully handles invalid or missing keys
• instance method IDBSideloader.clear() clear the given DB of all entries, as expected
• instance method IDBSideloader.deleteKey(key) selectively deletes the given DB entry as expected
  • gracefully handles invalid or missing key