Allow to open a custom file viewer window without opening the default one

[fwang2002]

I want to implement an extension to display a graphical presentation for a binary file. I have had a look at markdown extension. But it displays the customized preview window with the default editor window side by side. So I want to know:

1. Is there a method to only open the preview window (customized) without opening the default editor window.
2. Can the default editor window be embedded in a customized window
   It will be great if extension developers can freely compose their customized window with the internal VSCode views such as Editor, Tab, etc.

[yzhang-gh]

By now, there are several extensions to add support to more file types, (updated: 25 March, 2019)

Extension	#installs
Excel viewer	379,793
PDF viewer	136,801
NuGet NuPkg Viewer	8,786

We can see the great needs of this feature. But all of those preview extensions have a limitation.
If we open a .pdf or .xlsx file by directly clicking on the file in the vscode file explorer (just like how we open a text file), vscode will complain it's a binary file and won't open it.
To preview the file, we need to right-click the file and select 'open preview'.

So, besides registerTextDocumentContentProvider, having another command such as registerFileContentProvider would be great.

[chengs]

Support @neilsustc.

I tried to improve the pdf previewer and found that it was impossible because of lacking related extension hooks.

Indeed, vscode already has built-in schemes for reading images and videos in src/vs/base/browser/ui/resourceviewer/resourceViewer.ts. It would be great to design a similar extension hook.

[chengs]

Ref to #10821

[yzhang-gh]

I think we should guide more people to this issue and give more 👍 on it.

It was after I tried many keywords that I finally found this issue.