Upgrade to PostCSS 8

[yhatt]

PostCSS 8 is coming now! PostCSS is the most important part for Marpit's theme support.
https://github.com/postcss/postcss/releases/tag/8.0.0

PostCSS 8 has a new visitor API. It will no longer need to walk through AST again and again by a lot of Marpit build-in plugins, and we can expect an improved performance to transform themes.

In addition, PostCSS v7 had prevented bundling Marpit by rollup in depending projects, Marp CLI and Marp for VS Code, due to the circular dependency. Actually marp-team/marp-vscode#124 has reported the slow down of VS Code caused by not bundled modules.

Marpit v2?

A breaking change we have to take care is the end of support for EoL Nodes. Marpit also has no longer tested against EoL Node 8 but still keeping support while Marpit v1 as a matter of courtesy. If upgraded PostCSS to v8, Marpit would not be able to install in Node 8 environment even though Marpit supports it. So we must need to bump the major version to upgrade PostCSS.

However, Marpit v2 is still RFC in #194. We wanted to discuss about the future of Marpit framework, but no one had interested into v2 and we have passed a year with receiving no meaningful comments. 😢

To upgrade PostCSS into v8, we have to defer the plan of the original Marpit v2.