Allow users to use & as a delimiter to search and insert Epic references into Duo chat

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• prototype: https://www.figma.com/proto/uPU1dQRCyarQyvGRteLG19/GitLab-Duo-Chat-SSOT?page-id=2566%3A654&node-id=2577-7796&node-type=canvas&viewport=2095%2C1583%2C0.8&t=W0y92cl420uAETf0-1&scaling=scale-down&content-scaling=fixed&starting-point-node-id=2577%3A7796
• design file: https://www.figma.com/design/uPU1dQRCyarQyvGRteLG19/GitLab-Duo-Chat-SSOT?node-id=2577-7796&t=SRFdIv5hNB2Eh3Dk-4

Acceptance criteria

1. When a user is composing a message to Duo, if they start the message with a & or they type whitespace then a &, they should invoke the same Issue search/autocomplete that users see today when running the /include command and choosing Issue
2. If the user hits esc after invoking this menu, then the menu is dismissed and the & is treated like normal text in the user message
3. While focus is in the menu, things the user types (as in item 2 above) should not be added to the message they are composing
4. After the user has finished interacting with the menu invoked by the & delimiter:
   1. A token for the item the user selected should appear in the added context tokens row, as would happen today when the user uses the /include command
   2. the name of Epic the user selected should be inserted into their message after the & delimiter. The name should be a rich text link and hovering over it should show the same popover that would show on hovering over the associated token in the tokens row

See related issues for specific context delimiters:

• Allow users to use `#` as a delimiter to search... (#505658)
• Allow users to use `+` as a delimiter to search... (#505662)
• Allow users to use `!` as a delimiter to search... (#505659)