MarkdownPart: move to separate mdoc-parser module

[kitbellew]

scalafmt has an own markdown parser, to be able to format any scala code found in fenced blocks. In addition to duplicating functionality in that parser, it also doesn't support indented code blocks.

Let's instead extract the parser portion of the mdoc logic into a module so that it can be re-used. That module will have no dependencies.

Helps with scalameta/scalafmt#3672.

[kitbellew]

@tgodzik fyi, in case you missed this :)

[tgodzik]

One minor thing is due to the last update we need to release on JDK 11 now.