zcbor.py: Add basic support for .default in code generation. #1499

Job	Run time
Merge test 1 - Twister (Windows) (mps2/an521/cpu0 - build only)	3m 53s
Merge tests 2 - Python () functional tests (Windows)	51s
Merge test 1 - Twister (Ubuntu) (native_posix)	1m 58s
Merge tests 2 - Python (3.8) functional tests (Ubuntu)	12s
Merge test 1 - Twister (Ubuntu) (native_posix with asserts)	1m 49s
Merge tests 3 - Check repo files (Ubuntu)	28s
Merge tests 2 - Python (3.9) functional tests (Ubuntu)	15s
Merge test 1 - Twister (Ubuntu) (native_posix/native/64)	2m 1s
Merge tests 2 - Python (3.10) functional tests (Ubuntu)	13s
Merge test 1 - Twister (Ubuntu) (native_posix/native/64 with asserts)	1m 46s
Merge tests 2 - Python (3.11) functional tests (Ubuntu)	12s
Merge test 1 - Twister (Ubuntu) (mps2/an521/cpu0)	1m 58s
Merge tests 2 - Python (3.12) functional tests (Ubuntu)	12s
Merge test 1 - Twister (Ubuntu) (mps2/an521/cpu0 with asserts)	2m 2s
Release tests 4 (Python ${{ matrix.python-version }}${{ (((matrix.asserts != '') && ' with asserts') \|\| '') }})	0s
Release tests 1 - Check versions	0s
Release tests 3 (Python ${{ matrix.python-version }}${{ (((matrix.asserts != '') && ' with asserts') \|\| '') }})	0s
Release tests 2 - Fuzz	0s
	17m 50s

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