Workspace files in Positron should be named `[workspace].positron-workspace`

Description

Currently in Positron, Workspace files are named [workspace].code-workspace. This conflicts with Visual Studio Code and, if you open one of them, it will open in Visual Studio Code instead of Positron. Positron should name its workspace files [workspace].positron-workspace and become the handler for that file type to avoid this conflict.

System details:

Positron and OS details:

Positron Version: 2025.05.0 (Universal) build 142
Code - OSS Version: 1.99.0
Commit: 17df8dd67df471d39688fc6cd02e3f23543b4d72
Date: 2025-05-05T15:42:05.662Z
Electron: 34.3.2
Chromium: 132.0.6834.210
Node.js: 20.18.3
V8: 13.2.152.41-electron.0
OS: Darwin arm64 24.5.0

Interpreter details:

Any interpreter.

Describe the issue:

Steps to reproduce the issue:

1. With Visual Studio Code and Positron installed on your system, save a Workspace file using File -> Save Workspace As.
2. Close Positron.
3. Find your recently-created workspace file (in Finder or File Explorer) and double click it to open it.
4. The workspace will open in Visual Studio Code instead of Positron.

Expected or desired behavior:

The workspace should be opened in Positron.

Were there any error messages in the UI, Output panel, or Developer Tools console?

Negative, Ghost Rider.

Start with File → Save Workspace As and the desktop file-association and external-opening configuration; trace how workspace filenames and double-click opens are handled. Done means saved files use the .positron-workspace suffix and open in Positron rather than Visual Studio Code.