[pg] Add Client.escapeIdentifier() and Client.escapeLiteral()

See:
https://github.com/brianc/node-postgres/blob/master/lib/client.js#L306
https://github.com/brianc/node-postgres/blob/master/lib/client.js#L324

types/pg/index.d.ts

@@ -115,6 +115,9 @@ export class Client extends events.EventEmitter {
     pauseDrain(): void;
     resumeDrain(): void;
 
+    escapeIdentifier(str: string): string;
+    escapeLiteral(str: string): string;
+
     on(event: "drain", listener: () => void): this;
     on(event: "error" | "notice", listener: (err: Error) => void): this;
     on(event: "notification", listener: (message: Notification) => void): this;