Enable autocomplete for issue/work item extensible reference, [issue: and [:work_item:

The following discussion from !181859 (merged) should be addressed:

• @digitalmoksha started a discussion: (+1 comment)

  We're also going to need a followup to add support for the autocomplete trigger, I noticed that doesn't work yet.

Autocomplete currently does not work for [issue: and [work_item: when those references are typed. They should work the same way [vulnerability: works.

These reference types were added in Add issue and work_item reference extensions (!181859 - merged)