fix: restore FFFClearCache command implementation

[kuishou68]

Summary

• add the missing require('fff').clear_cache() implementation behind :FFFClearCache
• wire the advertised all|frecency|files modes to the existing cache cleanup primitives
• add focused Lua tests covering the default, frecency, and files paths

Notes

• I couldn't run the Neovim/Rust test stack in this sandbox because nvim and cargo are unavailable here.
• The fix is intentionally narrow: it restores the missing command target and keeps the cache clearing behavior in one Lua entrypoint.

Closes #169

[dmtrKovalenko]

I don't really know if this is something people will be actually using. It just cleans in memory

[dmtrKovalenko]

idk about this test it is too mocky but I don't want to make the real test that cleans my db. We should either have a test that provides custom paths for databases and verifies they are cleaned or just get rid from this test