feat: add setCode for editor

[snowdream]

Please check if the PR fulfills these requirements

• [x] It's the right issue type on the title
• [x] When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• [x] The commit message follows our guidelines
• [x] Tests for the changes have been added (for bug fixes/features)
• [x] Docs have been added/updated (for bug fixes/features)
• [x] It does not introduce a breaking change or has a description of the breaking change

Description

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Thank you for your contribution to TOAST UI product. 🎉 😘 ✨