Books/viewlet refactor (#7208)

* add saved/untitled views to the books viewlet and provide save option to untitled

* addressed comments

* fixes after merge

* await on async methods

* reverted back

* await on promise

* added localize for books view names

* initial commit

* missed a file change

* changes to make the merges work part1

* fixes after merges 2

* getChildren to get all books

* chnages to address comments

* fsPromises instead of fs.readSync

* merged master

* replaced deprecated fs.exists async call with pathExists

* renamed method

extensions/notebook/package.nls.json

@@ -28,5 +28,8 @@
 	"title.configurePython": "Configure Python for Notebooks",
 	"title.managePackages": "Manage Packages",
 	"title.SQL19PreviewBook": "SQL Server 2019 Guide",
-	"books-preview-category": "Jupyter Books"
+	"books-preview-category": "Jupyter Books",
+	"title.saveJupyterBook": "Save Book",
+	"title.SavedBooks": "Saved Books",
+	"title.UntitledBooks": "Untitled Books"
 }