Implement fallback port

[CommanderRedYT]

Feature Description: Implement a fallback port for the webserver. The docs do not provide a easy way for installing with a different port.

• Additional context: Unfortunately, this plugin uses port 8123, which is commonly used for homeassistant. It requires joining the discord and/or digging through github / docs to figure out how to change a simple integer to something else.

It would be nice that the application would just use the next free port (and maybe create the config file for it so it persists?) so people who play minecraft at home can use this without any trouble.

[ZockiRR]

There actually is a property in the config file (configuration.txt): webserver-port
You should be able to set the port with that.