Release API 2.0

[eleanorjboyd]

Need: The Python Environments Extension API needs a stable 2.0 release before the environments experiment can be removed.

[edvilme]

1. Document PackageManager and EnvironmentManager capabilities

We expose PackageManager and EnvironmentManager as a way for ourselves and other people to contribute abstraction wrappers around managers (e.g., uv, conda, pip). However, we increasingly run into the issue of trying to figure out if a certain feature is available per manager, or in a specific version of the tool.
Managers should have a clear way of advertising their capabilities for each of the operations exposed by the API.

1.1. Create Inventory of available capabilities

As a first step for this, we should have a canonical inventory of all the capabilities supported by the managers in the API. This should live in a centralized file and allow us to easily adopt new functionality.

1.2. Identify opt-in and opt-out functionality

Afterwards, we should check whether each functionality is supported by the majority of the managers, and determine if its default behavior should be opt-in or opt-out. This allows us to minimize breaking changes on managers that are not updated to advertise their functionality.