Support Gradle's Isolated Projects feature in EnhancedPluginAdditions#rootProjectDirectory()

[PaintNinja]

Prerequisite for MinecraftForge/ForgeGradle#1083

Using the isolated project API recommended by Gradle instead of grabbing the settings directory as this is a more direct replacement - there are edge-cases where the settings directory may differ from the root project's directory.

[Jonathing]

Has that isolated project API changed since 9.0? I'd like for GU to maintain the back compat with it until 4.0.

[LexManos]

Its still incubating, but according to the docs its existed since 8.8, so we should be fine.

[PaintNinja]

I've checked the javadocs for 9.0.0 and current (9.7.1 at the time of writing) and the getIsolated().getRootProject() methods haven't changed, so this PR doesn't break compat with Gradle 9.0.0.