Peek: Add basic editing and saving capabilities for simple text and source code files

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### Description of the new feature / enhancement

Add simple editing and saving support for text and code files like .txt, .md, .env, .xml, .json etc. When previewing these files, users should be able to click into the text, make quick modifications, and save the changes directly within the Peek window without needing to launch a separate application.

### Scenario when this would be used?

Currently making small edits to a file requires launching an external editor, however the new notepad app has become quite slow and full IDEs like VS Code are overkill for this use case. Since Peek opens instantly and already supports syntax highlighting for these files, allowing inline edits and saves would be much faster and also really useful.

### Supporting information

_No response_

Start by locating Peek's preview window and the existing syntax-highlighting path for text and source code files. Define how editing, saving, unsaved changes, and save failures should behave before implementation. Done means supported files can be edited and saved directly in Peek without launching another application.

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