view: split web/view's main.ts #2084

Found in the review of #2057, which adds the experimental view. Paths refer to #2059's head.

Summary

web/view/src/main.ts is about 1,570 lines. It holds the camera, the workspace, highlighting, the edit queue, menus and the page shell, so little of it can be tested.

• Split it into modules such as camera.ts, highlight.ts, editing.ts (pure literal building and edit planning) and shell.ts.
• Separate the pure parts of compare.ts from the DOM.
• Make computing the changed files a pure function.

Doing this together with #2064, #2065, #2066 and #2067 makes those fixes testable.