Make objc2 optional when translating frameworks #3223

Job	Run time
Check MSRV	55s
Check formatting	9s
Lint macOS x86_64	3m 19s
Lint iOS ARMv7s	3m 14s
Lint visionOS Aarch64 simulator	3m 15s
Lint GNUStep + exceptions	1m 33s
Lint header-translator	41s
Lint test-assembly	35s
Test macOS 15 Aarch64	25s
Assembly tests	56s
Verify header translator output	2m 2s
${{ ((matrix.test && 'Test') \|\| 'Build') }} ${{ matrix.name }}	0s
Check framework features	0s
Test Compiler-RT	21s
Compiler UI	47s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v1.9	56s
Test GNUStep with libobjc2 v2.0	1m 15s
Test GNUStep with libobjc2 v2.1 on nightly	1m 13s
Test GNUStep 32bit	1m 34s
	23m 10s

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