Add `#[ts(optional)]` to struct #1432

Job	Run time
Run 'workspace' end-to-end test	0s
Run 'dependencies' end-to-end test	0s
End-to-end test example	0s
Check that README.md is up-to-date	0s
Test ts-rs with --all-features	0s
Test ts-rs with absolute TS_RS_EXPORT_DIR	0s
Test ts-rs with TS_RS_EXPORT_DIR	0s
Test ts-rs with --no-default-features	0s
Test ts-rs with --all-features on Windows	0s
	0s

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