[preset-env] release candidate for `v8`

[romainmenke]

Things to review :

• issue templates
• PR labeler
• are all plugins included in PostCSS Preset Env
• are all plugins included in the CLI
• are all plugins tracked in CSSDB
• are all plugins listed on CSSDB as a polyfill

[romainmenke]

• release new major of postcss-progressive-custom-properties
• update the version for postcss-progressive-custom-properties in each package that depends on it
• run npm update && npm install
• release all the other plugins
• prepare and release postcss-preset-env V8 release candidate