[API Proposal]: Add Yaml for StringSyntaxAttribute

### Background and motivation

Add `Yaml` likes `Json` for `StringSyntaxAttribute`, `Yaml` is widely used for k8s/CI-CD configurations etc.
Maybe we should also support `Yaml` likes `Json`

### API Proposal

```csharp
namespace System.Diagnostics.CodeAnalysis;

public sealed class StringSyntaxAttribute
{
public const string Yaml = nameof(Yaml);
}
```

### API Usage

```csharp
var configuration = """
config:
default: true
MD013: false
""";
var yamlContent = new YamlContent(configuration);
using var response = await httpClient.PutAsync("/config", yamlContent);

class YamlContent([StringSyntax(StringSyntaxAttribute.Yaml)]string content): StringContent(content, "application/yaml");
```

### Alternative Designs

_No response_

### Risks

_No response_

Start by locating System.Diagnostics.CodeAnalysis.StringSyntaxAttribute and reviewing how its existing syntax constants are declared and tested. Confirm the API proposal and its YAML naming and usage, then add the Yaml constant; done means the API and relevant tests or validation for the runtime change pass.

Written by the indexing model from the issue text.