[STYLE] Use `import type` for type-only imports

[kakasoo]

Style

Description

Project convention states "타입 간 의존성은 import type 사용", but the following files use import { } instead of import type { } for type-only imports:

File	Import
src/types/DeepDateToString.ts	import { DeepStrictUnbrand }
src/types/DeepStrictObjectKeys.ts	import { DeepStrictUnbrand }, import { Equal }
src/types/DeepStrictObjectLastKeys.ts	import { DeepStrictUnbrand }
src/types/RemoveAfterDot.ts	import { ElementOf }
src/types/StringToDeepObject.ts	import { StringType }

Fix

Replace import { X } with import type { X } in each file listed above.

How to verify:

npm run build:test && npm run test