chore: repository maintenance

[Cherry]

This PR does some repo maintenance in terms of code quality, dependencies, testing, and typescript.

TypeScript

• Tweaks target to es6 over es5. This reduces a few unnecessary polyfills that are not required in a Workers environment, such as generators. We should look at setting this even higher in future with ESM support and async/await.
• With this tweak, expanded the config to build a similar bundle as before with applicable module and moduleResolution.

Code Quality

• Re-enables prettier properly (by adding to devDependencies) and fixes the npm script that runs this
• With above, runs prettier on existing files, and tweaks a few minor formatting things
  • The prettier config already present in the repo was used, with one tweak to mimic cloudflare-docs-engine. It was run on all existing files

Dependencies

• Updates GitHub Actions workflows to latest versions for slightly faster runs
• Bumps all dependency versions
• Moves all @types dependencies to devDependencies, as these do not need to be in the production install tree
• Update dependabot config to newest version

Testing

• Removes Node.js 8.x from testing. This version went EOL in 2019, and isn't supported by a few of the dependencies we use now. Technically a breaking change, but not one I think that's impacting, since this library isn't really "used" in a Node environment anyway.
• Adds Node.js 14 and 16 to tests. Wwe should probably look at dropping Node < 12 support at some point, since these versions are now EOL
• Tweaked ava configuration for newer and better TypeScript support with @ava/typescript

Publishing

• Implements some of the suggestions from packaging improvements workers-sdk#4978:
  • Specifies a files option in package.json to only publish needed files to NPM. (No .github, .dependabot, etc. configuration)

Docs

• Fixes table of contents anchor links in the README. As a side note, this is now built-in to Github by default for any markdown files. We could probably remove this section in entirely and simplify the README.

Let me know if I can provide any more insight. I'm excited to see this repository getting maintained again! 😃