Make AssetAction::Ignore not copy assets to imported_assets

[BeastLe9enD]

Objective

Lets say I have the following .meta file:

(
    meta_format_version: "1.0",
    asset: Ignore,
)

When a file is inside the assets directory and processing is enabled, the processor will copy the file into imported_assets although it should be ignored and therefore not copied.

Solution

• I added a simple check that does not copy the assets if the AssetAction is Ignore.

Migration Guide

• The public ProcessResult enum now has a ProcessResult::Ignore variant that must be handled.

[github-actions]

It looks like your PR is a breaking change, but you didn't provide a migration guide.

Could you add some context on what users should update when this change get released in a new version of Bevy?
It will be used to help writing the migration guide for the version. Putting it after a ## Migration Guide will help it get automatically picked up by our tooling.

[BeastLe9enD]

@alice-i-cecile is this migration guide ok?:D I don't know what else to write

[alice-i-cecile]

Updated your PR description to amend the migration guide :) It's unlikely to be particularly useful to folks, but this adding enum variants is a semver breaking change in Rust.