Fext self-update

[Flacy]

Since pypi deleted the fext-cli repository. It is necessary to make a self-update of fext using a separate command, let's say self-update. A separate command is needed so as not to add unnecessary checks to the install command. Self-updating will work using the GitHub API. Wheel packages will be named the same as on pypi, so as not to deviate from the standards and not reinvent the wheel.