runeliteplus: adds preliminary Gradle support

runelite-mixins/build.gradle

@@ -0,0 +1,10 @@
+
+description = 'RuneLite Mixins'
+dependencies {
+    testCompile group: 'junit', name: 'junit', version:'4.12'
+    testCompile group: 'org.mockito', name: 'mockito-all', version:'1.10.19'
+    compileOnly group: 'org.slf4j', name: 'slf4j-api', version:'1.7.25'
+    compile project(':runescape-api')
+    compileOnly group: 'com.google.guava', name: 'guava', version:'28.0-jre'
+    compileOnly group: 'javax.inject', name: 'javax.inject', version:'1'
+}