Gradle: Add decompile task to projects (#1273)

* Gradle: Add decompile task to projects

* add ff as dependency rather than adding ti in buildsrc

* smh smh smh smh smh

* Add ff version in buildsrc build file

This doesn't work in ext as it officially isn't part of the project

* Add missing ''s

buildSrc/build.gradle

@@ -0,0 +1,7 @@
+repositories {
+    maven { url "https://raw.githubusercontent.com/runelite-extended/maven-repo/master" }
+}
+dependencies {
+    compile localGroovy()
+    implementation group: 'net.runelite', name: 'fernflower', version: '07082019'
+}