Improve ergonomics for regenerating snapshots

[talf301]

Currently when we use SnapshotTesting.jl we need to maintain a global variable indicating whether or not we're regenerating snapshots, and then manually set this variable back and forth when we make apparently breaking changes. This PR adopts some of the ergonomics from ReferenceTests.jl, getting rid of this global variable in each test suite and instead providing 2 options:

• An environment variable JULIA_SNAPSHOTTESTS_UPDATE indicating whether or not we force updates (defaults to false)
• When run in an interactive terminal, the test will behave in an interactive way. If the test fails, the user is shown the diff and asked if this is correct and they would like to update accordingly

We get rid of the regenerate flag from the API, so this is a breaking change and requires a major version bump.