add outline support

[ijesonchen]

When opening in markdown-editor, the outline sidebar is disabled, showed "The active editor cannot provide outline information.".
Will you add this feature?
Thanks.

[TheOneValen]

outline works here, I just have to reopen the outline pane everytime

[huyz]

@TheOneValen are you sure it works? Maybe you're talking about the built-in outline that show up to the left but inside of the text editor:

I believe that @ijesonchen is talking about the VScode outline panel that works with all types of files (not just markdown):

Like OP, I would prefer to re-use the VScode Outline panel if possible.

[huyz]

Dup of #5