chore: add .idea to .gitignore (#3169)

Jetbrains IDEs create a `.idea` dir in each repository to store configurations (like `.vs` and `.vscode`).

I'm aware that `.git/info/exclude` can be used as a localized .gitignore file or `~/..config/git/ignore` for global ignores, so I'll leave it up to the maintainers if this solution is ok.

## Changes:

- Added `.idea` to `.gitignore` to prevent this dir from accidentally being committed.