Add `require_trailing_commas` rule and fix formatting

[HeySreelal]

Changes made:

• Added the lint rule require_trailing_commas
• Fixed the linter problems and formatted the whole project.

Please let me know any further changes required.

[devoncarew]

Hi! Thanks for the PR. I suspect that @natebosch will review.

I see that this PR is for two different things (adding a lint and reformatting project files, and, addressing the need to have a way to dispose of a GenerativeModel client). We do want PRs to generally address just one thing in order to make review easier. I'd suggest reverting the formatting changes from this PR and filing it as a separate one if you'd like feedback on that (though I do suspect that we wouldn't want to start using require_trailing_commas in this repo).

[HeySreelal]

Thanks for the suggestions @devoncarew, I've removed the close method related changes from the PR as per the discussion on #66.

This PR is now only contains changes for clearing the formatting issues with linter rule require_trailing_commas.

[natebosch]

Sorry for the slow response. I don't think we want to merge PRs that only flip trailing commas.

When it's ready we'll use the new dart format style which inserts trailing commas where sensible - until then I don't think we want to enforce one style or the other.