Implement browserslist handling outside of core

[moonglum]

This will keep the same behavior as we have right now, just without using the browsers provided by core. This allows us to remove browserslist as a dependency in core. It also makes it easier to understand the choices one has, as the entire selection is implemented in one place and not distributed over two code bases. What is the behavior?

1. If the user sets browserslist: false in the config of the bundle, we will not load the browserslist even if it is in the file system.
2. If the user does not provide the browserslist option, we will load the browserslist from the FS
3. If browserslist is set to a string, we will load the browserslist environment with that name (https://github.com/browserslist/browserslist/tree/main#configuring-for-different-environments)

It includes one improvement over the current setup: It will raise an error if the user provided a browserslist name that is not defined in their browserslist.

[moonglum]

Simplified this PR to keep the current behavior.