This refactor will enable us to swap out the LocalFilePersister and
RemotePersister. The Persist method in LocalFilePersist must match what
is a requirement for the RemotePersister, which is the addition of the
context as a param, even though the LocalFilePersister doesn't work
with it.

These two persisters will be interchangeable at a later stage depending
on the environment setup.

This env var will be used to configure the remote persister. When this
env var is available and setup correctly, it will help configure the
module to swap out the LocalFilePersister with the RemotePersister.

This will parse the url, basePath and headers from
K6_BROWSER_SCREENSHOTS_OUTPUT. It will also help determine whether the
module should use a RemoteFilePersister or a LocalPersister.

This method will help determine whether to use the LocalFilePersister
or the RemoteFilePersister. It mainly depends on whether a valid
K6_BROWSER_SCREENSHOTS_OUTPUT env var has been setup.

This initialise a FilePersister (either a local or remote) depending on
the environment, which is dictated by K6_BROWSER_SCREENSHOTS_OUTPUT.

Use the interface instead of the LocalFilePersister throughout the
codebase so that we can easily swap it out for a different persister.

The packages will now own their own interfaces instead of borrowing one
that was implemented in storage.

The browser package is the work horse on initialising and setting up
the environment for the global test run, iteration or vu. The decision
on whether to create a Local or Remote persister should rest in the
browser package's hands, but leave the details of what and how the two
persisters do their job in the storage package.

This type helps encapsulate and helps document what the parsed types
and usages are from the K6_BROWSER_SCREENSHOTS_OUTPUT env var.

This explains what it is doing better than parseEnvVar.

Use a mock persister when there's no need for either a local or remote
persister in a test. These tests do not need to validate that a
screenshot was saved.