Add java-prettify submodule

Test: bazelisk build //:gitiles && tools/run_dev.sh

Change-Id: I107354498367ec22e977e91f08e7de70d04b8573

.gitmodules

diff --git a/.gitmodules b/.gitmodules
index b339fa4..ddfec8c 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,3 +1,6 @@
 [submodule "modules/jgit"]
 	path = modules/jgit
 	url = ../jgit
+[submodule "modules/java-prettify"]
+	path = modules/java-prettify
+	url = ../java-prettify