Add __marimo__/cache/ to Python .gitignore

[akshayka]

Reasons for making this change:

The marimo Python notebook maintains a cache of binary data in __marimo__/cache/, and it recommends all users add this directory to their .gitignore, since this data cannot be meaningfully versioned.

I am one of the core maintainers of marimo, I would like to make it automatic for our users who use GitHub to have .gitignore that works well with marimo.

Links to documentation supporting these rule changes: https://docs.marimo.io/api/caching.html#caching-variables-to-disk

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This PR has been closed because it was inactive for 180 days. If you want to continue working on it, please open a new PR.