fix: expand VS Code predefined variables in all path-valued settings

[Copilot]

• Add substituteVariables function to handle ${workspaceFolder}, ${workspaceFolderBasename}, ${userHome}, ${env:NAME}, ${pathSeparator} variables
• Extract resolvePathWithSubstitution private helper to avoid duplicated substitution+resolve logic
• Update resolveConfigPath to accept optional workspaceFolder for multi-root workspace resolution
• Apply variable substitution to all path-valued settings: rootConfig, workspaceConfig, nodeExecutable, terminalShellPath, vitestPackagePath, and debugOutFiles
• Add unit tests for substituteVariables in config.test.ts
• Apply oxfmt formatting fixes

[sheremet-va]

@copilot run pnpm fmt:fix command to fix formatting errors

[Copilot]

@copilot run pnpm fmt:fix command to fix formatting errors

Done in 7521241.