Sort assets that use the latest Bevy version first

[rparrett]

Motivation

Help users find up-to-date assets quicker

Fixes #760
Fixes bevyengine/bevy-assets#297

Solution

Sorts assets by the following key:

(
    // Any existing order defined in the bevy-assets metadata.
    // e.g. for "official" templates and learning material
    existing_order,
    // Uses the semver crate to compare the "version requirement" from the asset with
    // the latest Bevy version
    is_semver_compatible_with_latest_bevy,
    // If all else is equal, randomize
    random
)

[benfrankel]

Small nits.