Update index.d.ts and improve startDrawingMode first param type

[ARZarkesh]

Please check if the PR fulfills these requirements

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

Description

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