Implement `Var` and `Export` for `DynGd<T, D>` #3131

Job	Run time
clippy	1m 57s
doc-lints	39s
godot-itest (macos)	58s
godot-itest (windows)	3m 25s
license-guard	41s
run-examples	1m 18s
godot-itest (linux-full)	3m 0s
rustfmt	9s
godot-itest (linux-features-experimental)	1m 44s
unit-test	2m 19s
godot-itest (linux-4.1)	57s
godot-itest (linux-4.2)	57s
godot-itest (linux-memcheck)	2m 6s
ci-status	1s
	20m 11s

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