added a runUi gradle task, fixed README

[mfishma]

This is more of a proposal, since we didn't talk beforehand.

To use the new MappingUI, I wanted a quick way to run the server, but I don't use IntelliJ.

Here's what I updated:

• Added a runUi task in the build file so you can start the UI with one command, .\gradlew runUi.
• Updated MappingUi.java to look for ../ViaVersion & ../ViaBackwards which are needed to run, with all-lowercase or proper casing.
• Updated README: I couldn't find a reference to BlockStateMappingUi so I assumed that was outdated and also added the new Gradle instructions.