libobs-metal: Enable Swift language

[Nerixyz]

Description

This essentially relands #12691 which was closed (due to inactivity?). It explicitly enables the Swift language in the libobs-metal subdirectory. I added the original author as a co-author.

Motivation and Context

CMake only enables C and C++ by default (docs):

By default C and CXX are enabled if no language options are given.

Swift is enabled in the macOS camera extension:

obs-studio/plugins/mac-virtualcam/src/camera-extension/CMakeLists.txt

Line 18 in 5533a27

enable_language(Swift)

However, that isn't included when building OBS with ENABLE_PLUGINS disabled (e.g. when building out of tree plugins). When configuring, you'd get the following error:

CMake Error: CMake can not determine linker language for target: libobs-metal

How Has This Been Tested?

In GitHub Actions with this commit cherry-picked to 32.1.0. Before; After (the run still fails but the configure step works).

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.