v8.0.0

[evanshortiss]

Preparing for a v8.0.0 release.

I will push to the typescript-refactor branch in the next few days with a working codebase that handles some or all of these items:

• [x] Rewrite module in TypeScript
• [x] Support for non Node.js/React environments #155
• [ ] Deno support #153
• [x] Support a description function #142
• [x] Add option to make all variables required by default #147
• [x] Update documentation
• [ ] Create a migration guide for API changes from v7 to v8
• [ ] Support both ESM and CJS (looks like nearform/lyra is a good example of this)
• [x] Add docs for Next.js/React usage (issue #162)
• [x] Add docs for Vite

[evanshortiss]

Notes:

• Removed ability to override built-in accessors
• Set minimum engine version to Node.js 12.x
• Changed extension API. Now requires invoking the extension via usingExtension(fn)
• The accessors property is no longer attached to EnvInstance (the return value of from())
• Removed the default exported "convenience" logger. Developers now must create a logger using the exported function
• Changed the built in logger to perform a production mode check using a boolean instead of a "prod" string