Add a settings.gradle to ensure consistency (FabricMC#1929)

If no settings file is found, Gradle will assume the project could be a
subproject and will scan any containing directories for a
settings.gradle. This can cause issues if such a file exists, which can
happen when using a composite build to set up a local development
workspace.

settings.gradle

@@ -0,0 +1 @@
+rootProject.name = "yarn"