feat(plugins/leetcode): add plugin (#1213) [skip ci]

source/plugins/leetcode/queries/problems.graphql

@@ -0,0 +1,18 @@
+query Problems ($username: String!) {
+  allQuestionsCount {
+    difficulty
+    count
+  }
+  matchedUser(username: $username) {
+    problemsSolvedBeatsStats {
+      difficulty
+      percentage
+    }
+    submitStatsGlobal {
+      acSubmissionNum {
+        difficulty
+        count
+      }
+    }
+  }
+}