Closes #647

Pull Request Overview

Adds documentation guidelines for upcoming Swift language features that can be enabled via feature flags before becoming default. This establishes a standard format for documenting experimental or preview features.

• Adds a new "upcoming features" section to the style guide
• Provides template language for feature documentation with version and identifier information
• References SE-0362 implementation for piecemeal adoption of language improvements ---

**Tip:...

The template note contains placeholder text 'n' and 'SomeFeatureIdentifier' that should be explained or formatted differently. Consider adding clarification that these are placeholders to be replaced with actual values, or use a more obvious placeholder format like {version} and {feature-identifier}. ``suggestion > **注:** 以下の {version} および {feature-identifier} は、実際の Swift のバージョン番号および機能識別子に置き換えてください。 > 注: この言語機能は Swift {version} の一部となります。現在のバージョンの Swift でこの機能を有効にするには、機能識別子 {feature...