Make web-test-runner and web-dev-server pure ESM packages

[web-padawan]

Motivation

Once Node.js 16 becomes LTS, it will become a third LTS version with native ES modules support.

Some maintainers have already started migrating their packages to be pure ES modules.
See the gist for an explanation of what it a pure ESM package.

In practice, it means that we drop support for CommonJS and drop support for older node versions (10.x).

Dependencies

Making @web packages pure ES modules would enable us to use dependencies that are pure ES modules too:

• cli-cursor@4 - fix(deps): update dependency cli-cursor to v4 #1641
• estree-walker@3 - fix(deps): update dependency estree-walker to v3 #1241
• is-stream@3 - fix(deps): update dependency is-stream to v3 #1610
• globby@12 - fix(deps): update dependency globby to v12 #1583
• log-update@5 - fix(deps): update dependency log-update to v5 #1728
• node-fetch@3.0.0 - chore: update node-fetch to 3.0.0 stable #1661

There might be other dependencies that get major bumps with dropping support for CommonJS in future.

Proposed solution

1. Update "exports" field to remove "require" entrypoints for all affected packages
2. Add "type": "module" to package.json to indicate packages are pure ES modules
3. Maybe also a few other tweaks, see How can I move my CommonJS project to ESM?

[Westbrook]

I like it! Would we also be in a place to use this change as he switch to 1.0?

[web-padawan]

The first step would be to bump minimal Node.js version: #1647

[web-padawan]

Some PRs that also not directly related but need to be merged before 1.0:

• mocha@9 chore(deps): update dependency mocha to v9 #1498
• ts-node@10 chore(deps): update dependency ts-node to v10 #1469