Proper text previews

Fixes #17828

* Modified the preview provider to provider smaller fonts for the
  previes (so it is not so screaming)
* Modified the sidebar to show plain text and markdown files full size.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>
Signed-off-by: npmbuildbot[bot] <npmbuildbot[bot]@users.noreply.github.com>