Deprecate NPM package in favor of lil-gui

[georgealways]

Hello!

I think lil-gui has reached a level of maturity where we can mark dat.gui deprecated and point users to lil-gui.

My proposal would be to publish a new version 0.8.0 and immediately mark it deprecated. This PR just bumps the version and modifies the README.

# after merge
git tag v0.8.0
git push --tags
npm publish
npm deprecate dat.gui@"^0.8.0" "This package has been deprecated and is no longer maintained. Please use lil-gui."

This will prevent a deprecation warning when installing old projects, only showing on fresh npm i dat.gui installs.

cc @mrdoob

[mrdoob]

LGTM!