yaml.schemas: document and support variable in schema paths

[chirag127]

Problem

The yaml.schemas setting requires absolute paths or URLs for local schema files. VS Code supports ${workspaceFolder} variable interpolation in many settings (e.g. launch.json, tasks.json, files.watcherExclude), but vscode-yaml does not resolve this variable. Users who work across machines (or share a committed .vscode/settings.json) must either hardcode an absolute path (breaks across machines) or use a schema registry URL (requires internet access).

Steps to Reproduce

1. Create .vscode/settings.json:

   {
     "yaml.schemas": {
       "${workspaceFolder}/schemas/my-schema.json": "*.yaml"
     }
   }

2. Open a .yaml file in that workspace.
3. No schema validation occurs; the Output → YAML channel shows:
   Unable to load schema from ${workspaceFolder}/schemas/my-schema.json

Expected Behaviour

The extension should resolve VS Code variable substitutions (${workspaceFolder}, ${workspaceFolder:name} for multi-root) before passing the schema URI to the language server. VS Code exposes these via vscode.workspace.workspaceFolders and the vscode.commands.executeCommand('vscode.resolveWorkspaceSetting') path.

Workaround

Users can use a relative path like ./schemas/my-schema.json, but this is neither documented nor consistently resolved across all YAML files in sub-directories.

Proposed Fix

In the configuration-reading code, after reading the yaml.schemas map keys, run each key through a variable resolver:

function resolveVars(schemaPath: string, folder: vscode.WorkspaceFolder): string {
  return schemaPath.replace(/${workspaceFolder}/g, folder.uri.fsPath);
}

Document the supported variables in the extension README and the package.json setting description.

Environment

• OS: Windows 11 + macOS (affects both — different absolute paths per machine)
• VS Code: 1.89+, shared .vscode/settings.json in git
• Extension: redhat.vscode-yaml latest