Simplify Marpit plugins by using injected instance into markdown-it instance

[yhatt]

Now, Marpit plugins require injected marpit member into markdown-it instance. We've updated plugins to use injected instance for reading Marpit options.

We think that rarely happen this case, but you won't have to care params when use Marpit plugins individually. Just inject Marpit instance to marpit member.

const markdownit = new MarkdownIt()
markdownit.marpit = new Marpit()
markdownit.use(marpitSlide)

If you were using marpit.markdownItPlugins interface, marpit.markdown and markdownit.marpit would be updated to the last injected markdown-it/Marpit instance.

const marpit = new Marpit()
const markdownit = new MarkdownIt().use(marpit.markdownItPlugins)
// (marpit === markdownit.marpit && markdownit === marpit.markdown) === true

The purpose of this change is to prepare building an escape hatch of deprecated options. By injecting opinionated feature by individual plugins instead of controlling plugin's feature by Marpit options, the power user may build custom Marpit from a plain markdown-it instance with just picking favorite plugins. However, there is not a desire like that at this moment, so we won't work splitting plugins actively now.