Fixes from a review of 0.4.0 before it reached a real Sublime Text; the
channel entry is unchanged. superfences handed every fenced block to
Pygments whenever another package had installed it, which turned a Mermaid
fence into plain code; Pygments is now off explicitly. The browser page's
<base> captured #heading links and sent them to the directory; relative URLs
are now resolved in the tree and headings carry the preview's own ids. Two
list-preprocessor mistakes -- a `>` line inside a fence, a heading at an
item's content column -- render as they did in 0.3.1. The libraries are no
longer imported at load, so a manual install without them gets a dialog
naming them and the fix instead of a package that fails to import. The
browser page is written private to the user and a browser that will not
start is reported with the page's path. ADR 0012 carries the amendments.