MMD3 compile - convert Scrivener links to MMD3 links

Since MultiMarkdown3 has internal cross-reference support, it would be very handy if there was some way to enable converting to this link type. For maximum handiness it would also need to check for section headings that share the same name, then add an auto-generated unique ID to them. (I’ve put an example below of what I mean.

This is a link to a [Section Header][].

## Section Header ##

This is a link to a [shared section name][shared1]. This is [another][shared2].

### Shared Section Name [shared1] ###

### Shared Section Name [shared2] ###