move shellcheck to local script (#364)

* move shellcheck to local script

* newline

* no -x

* shellharden

* shfmt
diff --git a/.shellcheck_tree b/.shellcheck_tree
new file mode 100644
index 0000000..633ffdf
--- /dev/null
+++ b/.shellcheck_tree
@@ -0,0 +1,2 @@
+# configuration for shellcheck_tree.sh
+excludes+=('third_party/*' 'build/*')