Fix motion todo example by removing mwc components

[binlabs]

The Motion: Todo List Transitions example has broken for several months now due to 404s that are occurring when pulling in the Material Web components that this example uses.

See #1429

I tried bumping the version of those packages but faced the same error.

In this PR I simply replaced the MWC elements with their native equivalents. I tried to make the minimum possible change to get this example working again.

Tested and working on Mac in recent versions of Firefox, Safari, Edge and Chrome.

I did have two questions:

1. This example was the only place in the entire repo that uses mwc checkbox, formfield, and textfield. Should I remove those 3 dependencies and regenerate the main package lock?
2. Should I add a little styling to make it look a tiny bit better (e.g. styling to make it more obvious what the remove button does?

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[binlabs]

It just occurred to me the reason why the MWC components weren't working - they probably just need to be updated to use the newer @material/web package.

If you'd rather I do that instead of use native elements like I did in this PR, let me know and I can make that change.

[justinfagnani]

Thanks for looking at this @binlabs!

I'm not sure why existing packages would break like this, that's unfortunate.

I personally would go for fewer dependencies so there's less likely to be a break like this in the future. We can add some minimal styles to make the native controls look nice for the demo (which maybe you did already).

@sorvell wdyt?