[vscode] Error message with vscode.open command with option to create missing file

[FernandoAscencio]

What it does

Adds user feedback when attempting to open a file that does not exist.
Partial implementation of 6568.
Closes 5667

How to test

1. Install this extension. Based on the one provided in 6568.
2. Run the Open Some File command.
3. Verify the error message appears.

Additional Information

The list below are some interesting things that were found during testing and more detail regarding the testing file open-some-file-0.0.9.

• Currently implementing a prompt to create the file is no issue, it just has not way of handling some of the test cases.
• The Open Wrong command passes two location. One of a file and one of a folder, both with the same name. Theia creates a empty folder while VS Code prompts the user to create a file.
• Open Nothing attempts to open a file with undefined URI.
• Open Not Folder attempts to open a file with a URI that has an empty string as path.
• Open Folder As File will open a folder as if it was a file.
• No feedback when an unregistered command is called.

Review checklist

• As an author, I have thoroughly tested my changes and carefully followed the review guidelines

Reminder for reviewers

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