Support System.Text.Rune

[Joy-less]

Since char is a 16-bit UTF-16 character, it doesn't support characters outside the BMP encoding range and need to be combined as surrogate pairs.
Rune is essentially a 32-bit UTF-32 character, and can support all Unicode characters, including emojis. It's wonderful.

This PR adds:

• ValueStringBuilder.Append(Rune)
• ValueStringBuilder.AppendJoin(Rune, IEnumerable<string?>)
• ValueStringBuilder.AppendJoin<T>(Rune, IEnumerable<T>)

This PR also optimises AppendLine(scoped ReadOnlySpan<char>) by avoiding an unnecessary string allocation. It also removes an unnecessary null check in AppendJoinInternalString<T>(ReadOnlySpan<char>, IEnumerable<T>).

[linkdotnet]

Hey @Joy-less
I love it - thank you very much for the PR and especially for fixing my stupid English grammar mistakes! Really appreciated.

One last bid:
Could you add a new ## Added section explaining what you did in the CHANGELOG.md.
This file is used for generating release notes.

[Joy-less]

One last bid: Could you add a new ## Added section explaining what you did in the CHANGELOG.md. This file is used for generating release notes.

Done! :)

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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