Implement `Var` and `Export` for `DynGd<T, D>` #3127

Job	Run time
doc-lints	49s
clippy	2m 10s
godot-itest (macos)	54s
godot-itest (windows)	2m 20s
license-guard	30s
run-examples	1m 0s
godot-itest (linux-full)	2m 21s
rustfmt	7s
godot-itest (linux-features-experimental)	2m 11s
unit-test	2m 16s
godot-itest (linux-4.1)	1m 12s
godot-itest (linux-4.2)	56s
godot-itest (linux-memcheck)	2m 15s
ci-status	1s
	19m 2s

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