Add keyboard shortcuts to focus previous/next message (A11y)

### Your use case

#### What would you like to do?
I'd like to add keyboard shortcuts to focus the previous/next message in chat (**not** to edit the previous/next message you sent)

#### Why would you like to do it?
Currently, the only way to navigate to a message via the keyboard is to use Tab/Shift-Tab. But this cycles through each read indicator, the React/Reply/Etc. popup, and avatar before finally focusing the previous/next message, which takes several presses. This is bad for accessibility - anyone who can't use a mouse for whatever reason will find it frustrating to interact with messages (and painful, if they have RSI).

#### How would you like to achieve it?
Either rebind the Up and Down keys, which currently navigate between *editable* messages, to instead navigate between *all* messages, or add a new pair of keyboard shortcuts for this function.

### Have you considered any alternatives?

_No response_

### Additional context

_No response_