Update project (#23)

* Fix issue #11 - Cannot find TextureTypeToString in scope AiTextureType.swift

* Remove custom pkg-config files for mac - libassimp.5.2.4 has working support

* Fix tests

* Update mint dependencies

* Update CI

* Update headers

* Add Swift Package Index shields

* Update macOS runner platform

* Update Linux CI to account for missing make

* Remove texture to string

* Make tests Linux compatible again

* Update README

* Fix linux CI cache

Merge pull request #10 from ctreffs/immutable

Make types more immutable

Fix access level

Merge pull request #8 from ctreffs/update-5.2

Update to Assimp 5.2.0

Update README

Merge pull request #5 from ctreffs/feature/embedded-textures

Embedded textures

Fix AiMatKey initializer

Fix pipelines and macOS homebrew problem

Improve mesh API

CI update