Minor Keybase GUI chat bug: improper Markdown termination of italics

On the macOS version of the Keybase GUI (6.0.4-20230113191846+6e1343b357), I found a very small bug on the Markdown processing.

When emoting with `/me`, by convention, the line is output in italics. However, if the _last_ element on the line is prefixed by `@` (i.e. a username, a channel name, etc.), this breaks the italics. What happens is that the trailing `_` will become part of the name being mentioned, thus further breaking _that_ functionality (the GUI will still think that `@` mentions a user or a team — when hovering over it, you still get the appropriate popup window asking for "User or team?" and suggesting a lookup _with the trailing `_`_, which of course will fail).

The same happens when italicising things, of course, not only when emoting.

Note that the issue only happens when you _terminate_ a sequence of italics with a `@` mention. It does _not_ happen if the `@` mention appears in the middle of the sentence. A quick workaround for this is just to add an extra space before terminating the italics (which won't be visible anyway)

Here is an image showing what I mean:


That should be easy enough to fix on your Markdown parser :-)

Start by locating the Markdown parser used by the macOS GUI and reproduce the case where an italicized line ends with an @mention. It is done when the closing italic delimiter is not included in the mention, while @mentions in the middle of italic text continue to work.

Written by the indexing model from the issue text.