Add file upload example and address issue #294 with solution.

[JuniperChicago]

• There exists an issue discussing the need for this PR
• I added new tests for the issue I fixed or built
• I used pnpm run commit instead of git commit

[staltz]

Hi @JuniperChicago, thanks for the initiative. Is this a work in progress PR? There are a couple things to do before merging:

• Squash all commits into one and use pnpm run commit
• Remove commented code
• Reformat code using prettier, like in the rest of the codebase
• Remove CSS styles that make this example differ from other examples
• etc (I'll review more, also Jan might review)

[JuniperChicago]

@staltz, I was doing work on a file uploader and encountered gaps in documentation and examples on successfully attaching files to http post operations. I thought a representative example might be useful. So this example contains:

• accept dragged and dropped files
• files can be added by using
• a list showing all files selected
• files can be uploaded to server also included in example
• use @cycle/http request and response streams
• use of @cycle/state