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

# 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:**
```bash
npm run build:test && npm run test
```

Review the five listed files under src/types and confirm each named import is type-only: DeepDateToString.ts, DeepStrictObjectKeys.ts, DeepStrictObjectLastKeys.ts, RemoveAfterDot.ts, and StringToDeepObject.ts. Change the imports to use import type, then run npm run build:test && npm run test; done means both commands pass and no listed type-only import remains as a regular import.

Written by the indexing model from the issue text.