[extensions] Add an initial set of extension variables (#7035)

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

packages/cli/src/config/extensions/variableSchema.ts

@@ -0,0 +1,30 @@
+/**
+ * @license
+ * Copyright 2025 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+export interface VariableDefinition {
+  type: 'string';
+  description: string;
+  default?: string;
+  required?: boolean;
+}
+
+export interface VariableSchema {
+  [key: string]: VariableDefinition;
+}
+
+const PATH_SEPARATOR_DEFINITION = {
+  type: 'string',
+  description: 'The path separator.',
+} as const;
+
+export const VARIABLE_SCHEMA = {
+  extensionPath: {
+    type: 'string',
+    description: 'The path of the extension in the filesystem.',
+  },
+  '/': PATH_SEPARATOR_DEFINITION,
+  pathSeparator: PATH_SEPARATOR_DEFINITION,
+} as const;