Notifications optional toggle to disable mentions on mxid localpart only, but retain mentions

### Your use case

#### What would you like to do?
I would like to not be mentioned/pinged when someone uses my mxid localpart (and not a mention).

#### Why would you like to do it?
I'm in a lot of matrix rooms, and my username happens to be a common english word, if i enable notifications on `"Notify when someone mentions using @displayname or @ mxid"`, I will also get mentioned when someone simply types out my mxid localpart.
There is no current way as far as I can tell, either with old notifications or beta notifications to retain mentions (and replies) actually creating a notification while disabling this setting.

#### How would you like to achieve it?
Ideally, it would be nice if the `"Notify when someone mentions using @displayname or @ mxid"` setting actually worked the way it's described and not also notify you when someone just uses your mxid localpart, since a user can just easily add a keyword notification trigger for their own localpart if they wanted it to behave like this.
Alternatively, a optional toggle to disable this behavior would also be nice.

### Have you considered any alternatives?

I did find a way around this using keywords, by adding all possible formatting of mentions of my full mxid, but that system then prevent replies from working (since it seems that keywords only checks message body which in some scenarios of message replies will not contain any part of my username)

### Additional context

_No response_

Read the issue's notification-setting and keyword-notification examples first, then identify where Element handles plain mxid-localpart text, actual mentions, and replies. Done means the setting no longer alerts for an unmentioning localpart while genuine mentions and replies still notify, with the behavior verified in the relevant client tests.

Written by the indexing model from the issue text.