Support better multi-root workspace configuration

Discussed in https://github.com/redhat-developer/vscode-yaml/discussions/883

^{Originally posted by agreedSkiing February 2, 2023}
I work in a project where we support a platform that has configuration that uses the default redhat.vscode-yaml for formatting with their settings defined in each .vscode/settings.json and some newer configuration projects that use prettier for formatting and here is where I run into a problem.

When opening multiple projects in a VSCode which creates a multi-root workspace the redhat.vscode-yaml configuration stops working ☹, and since there is no standard set in our projects the have different indentations and use of “ and ‘.

So it would be a great improvement to support multi-root workspaces in a wider scope, since I can’t have a one config fits all here.

Start with Discussion #883 and reproduce the reported formatting failure in a VS Code multi-root workspace, using the per-project .vscode/settings.json files described there. The issue does not name implementation files or tests, so first identify the workspace-configuration entry points and agree on the expected behavior before making a change.