Add "Read Text Aloud" context menu action for messages

This merge request adds a simple "Read Text Aloud" menu action to the context menu for chat messages. When clicked, the message text will be read aloud using text-to-speech.

The intention behind this change is to make it easier for myself and other low-vision users to use Matrix, without needing to use a system-wide screen reader, in cases where the user still has enough sight left to navigate a computer faster without a screen reader.

I'd eventually like to have it read " said ," so that the message sender gets read aloud as well, but for now it just reads the plaintext message contents.

Another problem, at least on my computer specifically, is that the voice's accent doesn't seem correct. For whatever reason, on my system, messages are read in a Scottish accent which is harder for me to understand. Other apps don't do that, so I'm not sure what's going on there. I do not want to hardcode a specific voice/locale, since I want this feature to work well for everyone and not just me.

@teams/qa Please do break my code! :) - I've only tested with basic text messages.

@teams/usability Not sure if I put the context menu action in an ideal place, it's grouped in the same area as clipboard actions like "Copy Text."

@teams/localization How could I go about getting author names to be read aloud in a way that's properly translated for other languages? I'm not experienced with i18n.