Pressing backspace in composer at beginning of second line deletes a character on the first line

### Steps to reproduce

1. In a room, with the composer focused
2. Type some text e.g. "word1 word2"
3. Go back to halfway through the text and insert a line break (I did this by pressing Return, but by default you need to press Shift+Return) so now the text is "word1 \nword2" and your cursor is immediately before the w of word2.
4. Press backspace

### Outcome

#### What did you expect?
The newline to be deleted

#### What happened instead?
The w of word1 on the first line was deleted!


Note: I've noticed this a couple of times, for at least a week probably more, but thought I'd mis-typed.

### Operating system

Ubuntu 21.10

### Browser information

Firefox 99.0

### URL for webapp

https://develop.element.io

### Application version

Element version: 479d4bf64d97-react-fbbb9c27ba8f-js-4721aa1d241a Olm version: 3.2.8

### Homeserver

matrix.org

### Will you send logs?

No

Start by reproducing the issue in the web composer using the listed steps and inspect the composer’s backspace and line-break handling. Done means backspace at the beginning of the second line removes the newline rather than a character from the first line, with the behavior covered by an appropriate regression test.

Written by the indexing model from the issue text.