Switch from unpkg to jsdelivr CDN

[Balearica]

Tesseract.js uses the unpkg CDN to deliver important files by default (the worker code and core code), even if the user self-hosts the tesseract.js script. While this behavior can be changed, my guess is that most users rely on the default. It is therefore very important for the CDN to have 100% uptime.

Unfortunately, the CDN we use by default--unpkg--has experienced downtime multiple times in the last year, which has been reported by multiple users (see #727). Based on what I've read online, it looks like jsdelivr is more reliable as it uses multiple redundant CDNs.

https://www.jsdelivr.com/unpkg