Factory overrides

[ian-r-rose]

This adds a way for the end-user to override the default viewer for a given file type. I waffled for a while about whether this should be implemented on the file browser, the document manager, or the document registry. I went with the latter, since that handles all the logic for deciding which factories to use normally.

I am also thinking about adding a menu item to the "Settings" or "View" menu which exposes this for the markdown viewer specifically. Any thoughts about that?

This is a separate question from whether to make the markdown viewer default (which I would personally be in favor of), but should make it less pressing.

References

alleviates #6442, fixes #4048

Code changes

New function on the document registry to set an override for a given file type.
New user-settings in the docmanager-extension to expose this functionality.

User-facing changes

Users can set their own default renderer for a file type.

Backwards-incompatible changes

None, as far as I can tell.

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[ian-r-rose]

Ran into an issue with old typedoc configuration, so I also updated the tdconfig.json files to use es2017. That should be mostly noise for the purposes of review.

[ian-r-rose]

Now with an auto-generated help string for the schema using @afshin's transformer mechanism:

    // Overrides for the default viewers for file types.
    // Specify a mapping from file type name to document viewer name, for example:
    // 
    // defaultViewers: {
    //   markdown: "Markdown Preview"
    // }
    // 
    // If you specify non-existent file types or viewers, or if a viewer cannot
    // open a given file type, the override will not function.
    // 
    // Available viewers:
    // HTML Viewer    
    // Image    
    // CSVTable    
    // TSVTable    
    // JSON    
    // PDF    
    // Vega4    
    // Vega-Lite2    
    // Vega5    
    // Vega-Lite3    
    // Markdown Preview    
    // Notebook    
    // VDOM    
    // Editor
    // 
    // Available file types:
    // text    
    // notebook    
    // directory    
    // markdown    
    // python    
    // json    
    // csv    
    // tsv    
    // r    
    // yaml    
    // svg    
    // tiff    
    // jpeg    
    // gif    
    // png    
    // bmp    
    // html    
    // PDF    
    // vega4    
    // vega-lite2    
    // vega5    
    // vega-lite3    
    // vdom

[afshin]

This is awesome. Thanks, @ian-r-rose!

[vidartf]

Is the diff here accurate?