cmux module in coder registry

Adding a [registry](https://registry.coder.com/) module for cmux will make smooth adoption and increase awareness for `coder/coder` users. Should install cmux and add a `coder_app` to open in the browser. Remote (coder) workspaces [documentation](https://cmux.io/ssh.html#coder-workspaces) should mention this as the ideal path for installation.

Down the road, allowing the app to open cmux on the client using the remote workspace would be ideal. Might depend on extensions.

### Tasks
- [x] Publish cmux module
- [ ] parameterize default workspace
- [ ] parameterize install source
- [ ] parameterize base URL
- [ ] Update "coder workspaces" documentation
- [ ] Add cmux to [docs.coder.com](https://coder.com/docs/user-guides/workspace-access)