Add support for removing functions named `invariant` and assertions from `tiny-invariant`

Support `invariant` function and `tiny-invariant` library

tiny-invariant is a small, 0 dependency assertion library. The standard way of importing its assertion function is to name it invariant, which is why I added it to the variables config for babel-plugin-unassert.

[!NOTE] Technically this changes removes support for removing assertions from power-assert, because we're now overriding the default config. There is no internal usage of power-assert that I'm aware of, so not marking this change as breaking.

Why should we support `tiny-invariant`?

For dev-time assertions in the browser, usually we'd reach for the assert library. This is what Braid uses. However, it has an unnecessarily large dependency graph for an assertion library.

Hopefully we can eventually replace assert with tiny-invariant within Braid. Part of that involves supporting an alternative assertion library in sku. Not sure when the change in Braid will happen, as it depends on consumers using a version of sku that supports tiny-invariant (i.e. whatever version this change is released in).

Tradeoffs between `assert` and `tiny-invariant`

assert supports checking non-primitive values, has more useful error messages, and offers various assertion functions. tiny-invariant just asserts on a user-provided condition, throwing an error if it fails. That's it. This is likely suitable for most use cases, and will probably work just fine in Braid and most front-end apps/libraries.

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seek-oss / sku