Add sanitize script

[camillobruni]

This script will eventually replace the benchmark-specifics overrides we already employ for some workloads.

• clamp all setTimeout to 0ms delay to limit effects of CPU throttling
• Make requestIdleCallback and cancelIdleCallback noops
• Set a fixed userAgent string to avoid running potentially browser-specific code

The sanitize.js script is supposed to be included in all benchmarks.

[camillobruni]

Using the sanitize script will happen in a separate PR, see #267 as an early example.

[camillobruni]

As per meeting discussion, this needs some more detailed investigation on which workloads use which of these APIs. Happy to land the non-controversial stuff first and set everything up for more common scripts we want to share between workloads.