MacDown does not utilize the net.daringfireball.markdown UTType when copying data onto the pasteboard

If you copy out of MacDown's editor, the content is put onto the pasteboard as `public.utf8-plain-text`, eg:

```
Item 789514: public.utf8-plain-text: {length = 25, bytes = 0x23232320 48656164 65720a0a 54686973 ... 69732066 696e652e }
```

If you copy out of MacDown's preview, the content is put onto the pasteboard as rtf, html, et al:

```
Item 789514: com.apple.webarchive: {length = 1412, bytes = 0x62706c69 73743030 d101025f 100f5765 ... 00000000 0000054a }
Item 789514: public.rtf: {length = 496, bytes = 0x7b5c7274 66315c61 6e73695c 616e7369 ... 73206669 6e652e7d }
Item 789514: public.html: {length = 1146, bytes = 0x3c683320 69643d22 746f635f 30222073 ... 696e652e 3c2f703e }
Item 789514: public.utf8-plain-text: {length = 21, bytes = 0x4865616465720a0a546869732069732066696e652e}
Item 789514: com.apple.WebKit.custom-pasteboard-data: {length = 32, bytes = 0x01000000 07000000 0166696c 653a2f2f ... 00000000 00000000 }
Item 789514: public.utf16-external-plain-text: Promised
```

In both cases, MacDown should be placing content onto the pasteboard using the net.daringfireball.markdown UTI (encoded as UTF-8) as that is the official UTI for Markdown (cf: https://en.wikipedia.org/wiki/Markdown)

Start by locating the editor and preview copy paths that populate the macOS pasteboard, then inspect how their existing text, RTF, HTML, and webarchive representations are registered. Verify the net.daringfireball.markdown UTI is provided with UTF-8 Markdown data for both paths, and confirm the pasteboard exposes it alongside the existing formats.

Written by the indexing model from the issue text.