Typescript 3.x drop support for WebKit* Types

[eneufeld]

The File-tree-model is using WebKitEntry, WebKitFileEntry and WebKitDirectoryEntry eg https://github.com/theia-ide/theia/blob/d656a7abfc42346a4080e7f8665a7b42bf6fc7db/packages/filesystem/src/browser/file-tree/file-tree-model.ts#L181.
This is not compatible with Typescript 3.x as they removed WebKit* types see microsoft/TypeScript#25515 (comment)
So instead of WebKitEntry etc FileSystemEntry etc from the File API should be used (https://developer.mozilla.org/en-US/docs/Web/API/FileSystemEntry) .

[AlexTugarev]

@akosyakov already start working on that in #2563

[akosyakov]

We can extract this part from #2563 and migrate to TS 3 without project references for now.

@eneufeld I will look again, but I've not seen FileSystemEntry in new lib.d.ts. I don't want to rewrite the code against another API, since it does work and I was not able to find the complete standard equivalent, so just copying WebKit typings for now.

[eneufeld]

All solutions would be fine for me, right now I set the typescript version to 2.9.2 so I just wanted to report the error, so that others don't run in the same issues as I did