feat: add create issue API

lib/linear_mutations.rb

@@ -0,0 +1,16 @@
+module LinearMutations
+  def self.issue_create(input)
+    graphql_input = input.map { |key, value| "#{key}: \"#{value}\"" }.join(', ')
+    <<~GRAPHQL
+      mutation {
+        issueCreate(input: { #{graphql_input} }) {
+          success
+          issue {
+            id
+            title
+          }
+        }
+      }
+    GRAPHQL
+  end
+end