Returns true if the value is a string marked as Vim's string constant format.

import { exprQuote, isExprString } from "https://deno.land/x/denops_std@v6.3.0/helper/expr_string.ts";

console.log(isExprString(exprQuote`foo`)); // outputs: true
console.log(isExprString("foo")); // outputs: false