Support `${workspaceFolder}` and variable substitution in `pandoc.*OptString` settings

[ayoussef-insight]

Currently, the vscode-pandoc extension does not expand VS Code variables like ${workspaceFolder} in settings such as pandoc.docxOptString or pandoc.pdfOptString.

For example:

"pandoc.docxOptString": "--reference-doc=${workspaceFolder}/templates/reference.docx"

results in Pandoc receiving the literal string ${workspaceFolder} rather than the resolved path.

This makes it difficult to maintain portable workspace configurations or share templates located in a fixed subfolder.

Expected behaviour

VS Code variables (e.g. ${workspaceFolder}, ${fileDirname}, etc.) should be expanded before invoking Pandoc, matching how variable substitution works in tasks.json and launch.json.

Actual behaviour

Variables are passed literally to Pandoc.

Environment

• VS Code: (your version)
• vscode-pandoc: (extension version)
• OS: (Windows/macOS/Linux)
• Pandoc version: (output of pandoc --version)

Why this matters

This would let users:

• Keep relative paths in project configuration.
• Avoid hard-coded absolute paths.
• Make multi-workspace and team setups cleaner.

Suggested solution

Use VS Code’s [variable resolver API](https://code.visualstudio.com/api/references/vscode-api#workspace) (e.g. vscode.workspace.getConfiguration() and vscode.workspace.workspaceFolders) to resolve variable placeholders in strings before invoking Pandoc.