[RFC]: add `ndarray/base/scalar-dtype`

### Description

This RFC proposes to add `ndarray/base/scalar-dtype` that encapsulates the logic for resolving a default ndarray data type from a scalar value.

The scalar-to-dtype resolution logic is currently duplicated across multiple packages (e.g., `ndarray/base/atleastnd`, `ndarray/from-scalar`) and should be extracted into a dedicated reusable package.

### Related Issues

- TODO comment in `ndarray/base/atleastnd/lib/main.js`

> ```javascript
> // For scalar values, resolve a corresponding ndarray data type...
> if ( isNumber( v ) ) { // TODO: consider abstracting this logic to an `ndarray/base/scalar-dtype` (???) package, as this logic is found elsewhere (e.g., `ndarray/from-scalar`) and it would be good to avoid duplication, especially as we add support for more ndarray data types
> ```

### Questions

No.

### Other

No.

### Checklist

- [x] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [x] Searched for existing issues and pull requests.
- [x] The issue name begins with `RFC:`.

Start by reading the TODO in ndarray/base/atleastnd/lib/main.js and compare the duplicated scalar-to-dtype logic in ndarray/from-scalar. Define the reusable ndarray/base/scalar-dtype package around that shared behavior, with completion meaning the relevant packages can use one consistent resolution implementation.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.