Add a save icon for the Copilot Chat debug view (#471)

* Add command to save current chat log as Markdown

This commit introduces a new command to save the currently opened chat log to a file in Markdown format. It includes a dialog for selecting the save location and handles errors during the save process.

| File                                      | Changes Made                                      |
|-------------------------------------------|--------------------------------------------------|
| src/extension/log/vscode-node/requestLogTree.ts | Added `saveCurrentMarkdownCommand` and its implementation |

* Enhance save current chat log command in requestLogTree

- Improved command to save the currently opened chat log by accepting a resource from menu invocation.
- Added fallback to the active editor's document if no resource is provided.
- Updated logic to read text from the virtual document URI explicitly before saving.

| File                                   | Changes Made                                      |
|----------------------------------------|--------------------------------------------------|
| src/extension/log/vscode-node/requestLogTree.ts | Enhanced command functionality and error handling |

* Refactor save dialog to use .md extension for Markdown files

- Changed default filename extension from .copilotmd to .md
- Updated save dialog filters to reflect the new .md extension

| File                                   | Changes                          |
|----------------------------------------|----------------------------------|
| src/extension/log/vscode-node/requestLogTree.ts | Updated filename and filters for save dialog |