[i18n] Add Japanese translations to VS Code extension

[shimotmk]

Motivation for the change, related issues

Part of #2202

Add Japanese translations to VS Code extension

[Copilot]

Pull Request Overview

This PR adds Japanese translations for the VS Code extension documentation page as part of the internationalization effort. The translation covers the complete documentation including setup instructions, key features, and usage information.

• Translates the VS Code extension documentation page from English to Japanese
• Maintains all original structure and formatting with bilingual comments
• Preserves all technical references and links to external resources

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[Copilot]

Missing space between 'extension' and 'Start' in the commented English text. Should be '# VS Code extension\nStart a zero-setup development environment...' with proper line break or spacing.

Suggested change

	# VS Code extensionStart a zero-setup development environment using the [VS Code extension](https://marketplace.visualstudio.com/items?itemName=WordPressPlayground.wordpress-playground), and develop your plugin or theme locally without installing Apache or MySQL.
	# VS Code extension
	Start a zero-setup development environment using the [VS Code extension](https://marketplace.visualstudio.com/items?itemName=WordPressPlayground.wordpress-playground), and develop your plugin or theme locally without installing Apache or MySQL.

[fellyph]

Thanks @shimotmk for your contribution