migrate diff viewer to use monaco's diff viewer

[bcolloran]

monaco editor comes with the diff viewer used by vscode. this is obviously very actively maintained and quite polished, and migrating would also remove a dependency which is always nice. we should migrate to it when we can.

[bcolloran]

little more on this: i'm not sure how involved this will be; it might be tricky or it might be pretty easy.

we'd need to create a custom component to instantiate the monaco diff viewer, along the lines of the editor component but i think waaaay simpler since the state updating would not need to be so sophisticated -- i think we could write it as it's own component that could just take the same props as ReactDiffViewer and have it be a drop-in replacement, but i'm not 100% sure.

[wlach]

FWIW, our usage of react-diff-viewer is very very simple (as is the component):

iodide/src/editor/components/modals/revision-diff.jsx

Line 67 in dbfa999

<ReactDiffViewer