Implement `Var` and `Export` for `DynGd<T, D>` #3136

Job	Run time
doc-lints	44s
clippy	1m 58s
godot-itest (macos)	50s
license-guard	33s
godot-itest (windows)	2m 48s
run-examples	1m 19s
rustfmt	12s
godot-itest (linux-full)	2m 49s
godot-itest (linux-features-experimental)	1m 48s
unit-test	2m 21s
godot-itest (linux-4.1)	57s
godot-itest (linux-4.2)	59s
godot-itest (linux-memcheck)	2m 15s
ci-status	1s
	19m 34s

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