Heat oil in a large skillet over medium-high heat. Add white part of green onion and cook until fragrant and starting to brown slightly, about 1 minute. Stir in corn, edamame, sweet pepper, marjoram, salt, and black pepper and cook, stirring often, until the vegetables are crisp-tender, 4 to 5 minutes. Stir together broth and cornstarch; stir into corn mixture with remaining green onions. Cook and stir until thickened, bubbly, and liquid almost evaporates, 1 minute. Remove from heat and stir in butter until melted.