Add Golang project settings

Author: o-p

go.mod

@@ -0,0 +1,8 @@
+module github.com/o-p/leetcode
+
+go 1.15
+
+require (
+	github.com/stretchr/testify v1.7.0
+	golang.org/x/oauth2 v0.0.0-20210113205817-d3ed898aa8a3 // indirect
+)