[PowerPages][create-site] Preview and Edit Site Page and Command Registration

[amitjoshi438]

This pull request includes significant changes to the PowerPagesChatParticipant module to enhance the command registration and execution process, introduce a new command for creating sites, and implement a content preview feature. The most important changes are summarized below:

Command Registration and Execution Enhancements:

• Added registerCommands function to ChatParticipantUtils.ts to facilitate command registration.
• Updated PowerPagesChatParticipant.ts to utilize the new registerCommands function for command registration. [1] [2] [3]

New Command Implementation:

• Introduced the CreateSiteCommand class in CreateSiteCommand.ts to handle the creation of new Power Pages sites. [1] [2]
• Added POWERPAGES_COMMANDS constant in PowerPagesChatParticipantConstants.ts to define available commands.

Content Preview Feature:

• Implemented EditableFileSystemProvider in EditableFileSystemProvider.ts to manage file content for previewing site pages.
• Enhanced CreateSiteHelper.ts to include a previewSitePagesContent function that registers site pages for preview. [1] [2]

Constants and Utility Functions:

• Added constants for file extensions and encoding in constants.ts.
• Defined additional constants in CreateSiteConstants.ts for site creation parameters. [1] [2] [3]

These changes collectively improve the functionality and extensibility of the PowerPagesChatParticipant module, enabling better command handling and user interaction.