Define a shared ViewMode type for document viewer

[hoyla]

Follow-up from #613 (comment by @zekehuntergreen).

getDefaultView currently returns string | undefined. The actual set of view
mode values is finite and known:

• "text"
• "preview"
• "table"
• `ocr.${string}`
• `transcript.${string}`
• `vttTranscript.${string}`

We should define a shared ViewMode type and use it across:

• getDefaultView return type
• setResourceView parameter (currently any)
• SetResourceViewType Redux action (currently view: any)
• PreviewSwitcher navigate callbacks and PreviewLink props

This would catch invalid view strings at compile time and make the view mode
contract explicit.

Labels: document viewer, enhancement