Use contains from stdlib for CacheTable when available

[ire4ever1190]

I'm working on adding contains to std/macrocache in https://github.com/nim-lang/Nim/pull/21304 but the CI fails for this package since its already defined.

This just puts it behind a check so the original will still be used if unavailable

[Vindaar]

Oh, this is great, thank you! Does the declared(macrocache.contains) catch it if it's declared? I suppose it should, but I've only used NimMajor and friends for this kind of stuff before.

[ire4ever1190]

Yeah basically checks if the symbol can be found. And by prefixing with the modules name we can check if it exists in the module