Ensure list of packages and dependency graph in README is kept up to date

Currently, engineers must remember to run `yarn update-readme-content` whenever creating a new package or updating a package's internal dependencies. It would be great if they did not have to do this.

## Acceptance Criteria

- `yarn update-readme-content` takes a required `--check` option which exits with code 1 and a message recommending that `--fix` be used if the README is out of date
- `yarn update-readme-content` takes a required `--fix` option that autofixes the README
- `yarn lint` runs `yarn update-readme-content`
- `yarn lint:fix` runs `yarn update-readme-content --fix`

Start with the implementation behind `yarn update-readme-content` and the package scripts for `yarn lint` and `yarn lint:fix`. Add the required `--check` and `--fix` behavior, then verify that lint checks the README and lint:fix updates it, including the stated exit code and message when content is outdated.

Written by the indexing model from the issue text.