Add support for @ mentioning a user in the message input box

[kevinaboos]

Upon typing the @ character in a room's message input box, we should activate a pop-up that allows the current user to pick a mentioned user from the list of room members.
As the user types more characters, the typed content after the @ that matches a keyword will be used to filter the list of room members. Then, once the user selects a member to tag, that user's display name (as a proper linked username) should be inserted into the TextInput box. We should also support selecting a user via the Up/Down arrow keys and Enter.

Fortunately for us, Moly has already implemented this functionality, so we can just borrow their implementation. (I don't think it's been upstreamed into Makepad yet)
This has now been upstreamed into Makepad as the CommandTextInput widget: #278

In terms of visual design, I prefer the Element style of shown below. We should show the user's displayable name on the left and then the full user ID on the right of that, being sure to use ellipsis wrapping on both to gracefully handle the case when the popup window is too narrow to fully display long usernames/user IDs.

Examples

Here is how Discord shows the user-mentioning tag:

Here's how Element shows it:

[kevinaboos]

this is now much easier to implement thanks to the newly-contributed CommandTextInput widget now available in Makepad: makepad/makepad#621