BREAKING(cache/unstable): remove re-export in LruCache

[tomas-zijdemans]

MemoizationCache is defined in and documented as part of @std/cache/memoize. The re-export from @std/cache/lru-cache was maybe done as a convenience, but should be cleaned up:

• MemoizationCache is available from its own path (@std/cache/memoize) and the barrel (@std/cache)
• ttl_cache.ts also imports the same type but does not re-export it, creating an asymmetry.
• lru-cache.ts should export only LRU-specific symbols.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.41%. Comparing base (008db8d) to head (6541600).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7071   +/-   ##
=======================================
  Coverage   94.41%   94.41%           
=======================================
  Files         630      630           
  Lines       50486    50486           
  Branches     8946     8946           
=======================================
  Hits        47666    47666           
  Misses       2249     2249           
  Partials      571      571           

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