I did this by running:

```
npm init @eslint/config@latest
```

We know that we do not want to use flow anymore. We may want to adopt
TypeScript, but it is unclear if we will in the near term.

#593

I want to re-introduce ESLint, and instead of setting up ESLint to be
able to parse flow syntax, I decided that we should just remove it.

#608

I removed these annotations by running the following command:

```
npx flow-remove-types lib/ -d lib/
```

Then I formatted the files by running prettier on everything:

```
node_modules/.bin/prettier -w lib/
```

I think if we wanted to introduce TypeScript that it might be easier to
do it from scratch anyway instead of attempting to convert the flow
types over to a different type system.

TypeScript pretty much won, and we've removed Flow annotations from this
repo.

For convenience.

This is a node project, not a browser project, so we need to use the
node globals. And we also need to tell ESLint about our tests, and how
to ignore the stuff that used to be in the .eslintignore file.

This resolves most of the errors.

Now that we have ESLint set up, we need to fix the things it complained
about. This handles all of that in one commit.

This will help us avoid committing errors.

This will make it easier for folks to catch errors as they work.