LibRegex: Searching for anchored patterns like /^b/ should fail early if possible

[Andreas Kling]

If we're looking for /^b/ in the string aaaaaa (and multi-line mode is not enabled), we should be smarter than to scan through every single a before bailing.

Reduction from https://fragrantica.com/

const mb = 100;
console.log("Create " + mb + "MB string");
let s = "a".repeat(mb * 1048576);

let re = /^b/
console.log("Search for " + re + " in string");
for (let i = 0; i < 10_000; ++i) {
    console.log(re.test(s));
}