tree printer FEATURE basic implementation of YANG tree format printer

Follows RFC 8340.

Fixes #858
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 438913a..c081e35 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -132,6 +132,7 @@
     src/printer_schema.c
     src/printer_yang.c
     src/printer_yin.c
+    src/printer_tree.c
     src/plugins_types.c
     src/plugins_exts.c
     src/xml.c