✨ Extension method to convert a string to `ReadOnlySpan<Rune>`

**Is your feature request related to a problem? Please describe.**
`string` type has implicit conversion to `ReadOnlySpan` but no implicit conversion to `ReadOnlySpan`.

This could be handy when dealing with text that may content emojis and might be more robust than the naïve `char` approach

**Describe the solution you'd like**
A way to convert `string` or `ReadOnlySpan` to `ReadOnlySpan`

**Describe alternatives you've considered**
N/A

**Additional context**
Some articles that might help getting started :
- [Character encoding in .NET](https://learn.microsoft.com/en-us/dotnet/standard/base-types/character-encoding-introduction)
- [How to correctly count the number of characters of a string](https://www.meziantou.net/how-to-correctly-count-the-number-of-characters-of-a-string.htm)