feat: add AdapterEnv interface for ambient extension

The `AdapterRequestContext#env` method uses this interface to provide type safety at compile-time. It allows the user to make certain assumptions about the existence of particular environment variables.

packages/base/core/index.d.ts

@@ -1,3 +1,10 @@
+/**
+ * Interface defining the shape of environment variables
+ */
+export interface AdapterEnv {}
+
+export type AdapterEnvKey = keyof AdapterEnv | (string & {});
+
 /**
  * Request context as dispatched by the platform adapter
  */
@@ -24,7 +31,9 @@ export interface AdapterRequestContext<P = unknown> {
 	 *
 	 * @returns The value of the variable or undefined if it doesn't exist.
 	 */
-	env(variable: string): string | undefined;
+	env<K extends AdapterEnvKey>(
+		variable: K,
+	): K extends keyof AdapterEnv ? AdapterEnv[K] : string | undefined;
 	/**
 	 * Signal that the request hasn't been handled and the returned response is
 	 * a placeholder (usually a 404). In this case the adapter should handle the