Browser WebExtension / Add-on as additional platform

**Is your suggestion related to a problem? Please describe.**
Currently the only way to serve Element Web without having to maintain a Webserver in some form is installing Element Desktop

**Describe the solution you'd like**
An alternative could be bundling Element as [WebExtension](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions), which would have some benefits:

- Updates to Element can easily get deployed to users over official WebExtension channels (Firefox AMO, Chrome Store)
- Allows users to install and run the whole thing locally, without having to worry that their Element Webserver of choice does malicious MITM things
- It would allow to make the client installation reproducible, since the whole WebExtension package could be downloaded and hash checked before installing
- In combination with Native messaging it would allow to have host platform native features: #14523
- Since its a WebExtension it would open up a new set of possibilities with the [WebExtension API](https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API)

**Describe alternatives you've considered**
Not aware of any

**Additional context**
This already [exists unofficially](https://addons.mozilla.org/firefox/addon/radical-web), but I think it might be worth to consider doing something like that officially given the benefits.