feat: UploadsList components

[travis]

first pass at UploadsLists headless components. needs an example in react-ui to prove out

two main questions for reviewers:

1. any thoughts on the API for this component? feel free to pass on evaluating this until I have an example/docs
2. is it worth adding a "prev" function to the UploaderProvider so that we could build out a real basic "paging" component? I don't think we'll be able to add current page/total number of pages controls and indicators without more support from the backend service, but just forward and backward is better than a one way street
3. otoh, should I maybe add an infinite scroll list component to show off what we can do with the current API?

TODO:

• get feedback on API
• add example in react-ui
• add prev upstream to allow us to build basic pager component (added add prev to UploadsListProvider context value #143 for this)
• add infinite scroll list subcomponent (added add headless infinite scroll component to @w3ui/react-uploads-list #144 for this)

[codesandbox-ci]

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Latest deployment of this branch, based on commit 40665e7:

Sandbox	Source
@w3ui/example-react-file-upload	Configuration
@w3ui/example-react-sign-up-in	Configuration
@w3ui/example-react-uploads-list	Configuration
@w3ui/example-solid-file-upload	Configuration
@w3ui/example-solid-sign-up-in	Configuration
@w3ui/example-solid-uploads-list	Configuration
@w3ui/example-vue-file-upload	Configuration
@w3ui/example-vue-sign-up-in	Configuration
@w3ui/example-vue-uploads-list	Configuration

[travis]

ok I'm going to merge this since it's a sub-PR and will get reviewed with #136 - I'm confident enough after chatting with folks about this that it's the right thing that it doesn't need a separate review