Test consumer file system API

[jrieken]

Refs: #48034

• any @bowdenk7
• any @alexandrudima

Complexity: 3

There is new API: vscode.workspace.fs. It allows to read, write, and manage files and folders. Test that

• the API is stable and works without enabling proposed API
• the API works as promised
• this sample shows to derive uris and how to convert bytes to strings: https://github.com/microsoft/vscode-extension-samples/tree/master/fsconsumer-sample

[jrieken]

fyi - use the vscode-dts-utility to use the latest version of vscode.d.ts and/or vscode.proposed.d.ts