Variable-scoped input shadowing

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Adds

• variable.shadow(name, [inputs, ]definition)
• variable.unshadow(name)

This will enable variable-scoped input shadowing, e.g.:

main.variable(true).define("val", [], 1000);
const a = main.variable(true).define("a", ["val"], (val) => val);
const b = main.variable(true).define("b", ["val"], (val) => val);
a.shadow("val", 100);
b.shadow("val", 200);
const mainVal = await main.value("val"); // 1000
const aVal = await main.value("a"); // 100
const bVal = await main.value("b"); // 200

[duaneatat]

README changes coming soon 🙏

[duaneatat]

This is ready to go now, we've vetted the changes and they work as expected @mbostock 🙏