Move the header id tracking into the renderer context rather than a global state. Since the RendererContext is
initialized for each execution of the marked parser, we can store the context of which header ids have been seen within
a single page there which allows us to run these parse interactions in parallel.
(cherry picked from commit a16a70ca74)
Extends the Markdown parser to support `loading`, `decoding`, and `fetchpriority` attributes for images via curly brace syntax.
(cherry picked from commit 46269be842)
