core(legacy-javascript): update to target Baseline

[connorjclark]

• Rather than use esmodules as the anchor for "legacy" JS, use Baseline
• Automate the creation of the polyfill modules to check for via core-js-compat. This made it feasible to add many more polyfills
• Add additional tests to validate that each polyfill being checked can be consistently detected. If it cannot (even w/o source maps), it is excluded
• Removes support for detecting core-js@2 polyfills. But the recently added warning if core-js@2 is detected remains.

This audit can now be described as: inform developers when they are polyfilling a feature that is widely supported across all major browsers, and provide an estimate for how much that costs (in terms of bytes / impact on metrics via byte efficiency).