Implement global `let` bindings #2397

Job	Run time
Devcontainer: ubuntu-latest/debug	14m 26s
Devcontainer: ubuntu-latest/release	15m 32s
Native: macos-13/debug	20m 48s
Native: macos-13/release	12m 31s
Native, no testing: windows-latest/release	10m 52s
	1h 14m 9s

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