Overhauled GUI
- single window, no more separate modals for each process
- combined Options tab with nested tabs for BuildTools and GUI/Program

Code clean-up + refactoring

Forgot the code that updates compilation target selections, this has been fixed.

Parallel processing to build multiple server jars at the same time
MAVEN_OPTS field for Maven arguments
Added toggle to open the output directory in system file explorer
A timer is now shown at the end of any successful BuildTools run to show how long it took.

Added support for the new "experimental" argument in BuildTools.

Now bundling FlatLaf for a not-terrible look and feel for the GUI.

Changed the path formatter to be less Windows-centric and now properly detect how it should be formatted for execution depending on detected OS.