[RFC]: add `symbol/to-string-tag`

### Description

This RFC proposes adding the package `symbol/to-string-tag`.

The package should be very similar to the following existing packages:

- `symbol/has-instance`
- `symbol/iterator`
- `symbol/async-iterator`
- `symbol/is-concat-spreadable`

The primary difference is that the package should export `Symbol.toStringTag` if it exists.

### Related Issues

None.

### Questions

No.

### Other

When adding this package, one should do the following:

- copy one of the existing packages mentioned above
- rename the copied package
- find and replace the package name (e.g., `has-instance` => `to-string-tag`)
- find and replace the variable name (e.g., `HasInstanceSymbol` => `ToStringTagSymbol`)
- ensure the copyright years are 2025
- update the descriptions according to the symbol's purpose (see MDN)
- update examples to reflect the symbol's purpose (see existing packages for inspiration)
- run tests, benchmarks, and examples to ensure that everything passes
- go back through and check that there are no copy-paste mistakes

### 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 inspecting the existing symbol/has-instance, symbol/iterator, symbol/async-iterator, or symbol/is-concat-spreadable packages and choose one as the template. Create the corresponding symbol/to-string-tag package, updating its description, examples, variable names, and 2025 copyright years. Run the package's tests, benchmarks, and examples; done means they pass without copy-paste mistakes and the package exports Symbol.toStringTag when available.

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