Implemented UI for dropped files

[ekgame]

This PR introduces a pad UI for displaying custom files dropped to the editor and available for access by Uiua code and some additional related changes for running the pad.

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Features and changes:

• New UI for displaying dropped files.
• Click on the file name to insert the code to load it (preserves inferring &fras, &frab ans ~).
  • The "loading code" is no longer inserted upon dropping the file. The rationale is that it was coded to work when the code was empty, and the code wouldn't be inserted if there was already code there - which is very confusing. Now the code is inserted at the cursor on click which should be much more pleasant UX.
• Click on the "x" button to remove the file.
• Now handles all dropped files, not just the first one.
• Added a button next to settings to upload a file via a prompt (useful for mobile users).

The way the file list updates is a little hacky - it listens for changes to the output state and re-renders every time it changes. I could not find a better way to do it, but it seems to work and handles the cases where files are created/deleted by running code.