[babel-preset] Allow customizing Babel assumptions and set loose~ish defaults

[developit]

This adds support for customizing the assumptions configuration passed to @babel/preset-env, which is a granular replacement for Babel's previous "loose mode" functionality.

I've also included a set of default assumptions, with comments above each explaining what they do. Some of them are clear-cut "we would never rely on that" assumptions (document.all), others I've tried to infer from the TS+JS I've scanned through via cs.github.com.

The three I'm most concerned about would be objectRestNoSymbols, setSpreadProperties and superIsCallableConstructor. These all have good runtime performance payoff if we can land them by default - the normal "strict" helpers do extra iteration that the "loose" helpers can skip. If anyone believes Shopify might currently be relying on this functionality, we'd want to remove them.