fix(NODE-6124): utf8 validation is insufficiently strict

[aditi-khare-mongoDB]

Note: this PR description is copied over from the 6.x branch fix.

Description

Outside of web, our toUTF8 function was insufficiently strict and allowed overlong encodings.

What is changing?

Change our functionality to use jsTextDecoder to double check utf8 input when a replacement character is detected.

Is there new documentation needed for these changes?

No.

What is the motivation for this change?

Drivers wide initiative to make UTF-8 validation strict and consistent.

UTF-8 validation now throws a BSONError on overlong encodings in Node.js

Specifically, this affects deserialize when utf8 validation is enabled, which is the default.

An overlong encoding is when the number of bytes in an encoding is inflated by padding the code point with leading 0s (see here for more information).

Double check the following

• Ran npm run check:lint script
• Self-review completed using the steps outlined here
• PR title follows the correct format: type(NODE-xxxx)[!]: description
  • Example: feat(NODE-1234)!: rewriting everything in coffeescript
• Changes are covered by tests
• New TODOs have a related JIRA ticket