bugfix: Remove additional whitespace from summary

The recent changes caused the spaces to no longer be removed properly due to the fact that we no longer operate on an actual value when printing one line summary.

I tried solving this multiple ways but:
- there is no sensible way to just copy any value
- using pprint.Tree as itnermediate representation doesn't work as those contain Iterator and are one use only

The easiest was to just replace newline with space and then deduplicate whitespace to save space on the online summary. This also makes it consistent across Scala 2 and 3

improvement: Also print nested arrays

refactor: Use normal dotty driver instead of interactive one

This unblocks us to use plugins in markdown.

improvement: Release interfaces with Java 8 compatibility

This allows us to use the newest version of mdoc in Metals, but we can still fall back to an older version behind the interfaces.

MarkdownPart: identify `mdoc` code fence modifier

chore: Bump Scala version to LTS version