Add binding for cell selection (microsoft#13417)

> @greazer it's actually more appropriate to mark the 4 of us as 'reviewer's instead of assignees. You should be the assignee as you're going to submit this PR. Whoops, I just wasn't paying attention.

Author: greazer

package.json

@@ -289,6 +289,16 @@
  "command": "python.datascience.gotoPrevCellInFile",
  "key": "ctrl+alt+[",
  "when": "editorTextFocus && python.datascience.hascodecells && python.datascience.featureenabled && !notebookEditorFocused"
+ },
+ {
+ "command": "python.datascience.selectCellContents",
+ "key": "ctrl+alt+\\",
+ "when": "editorTextFocus && python.datascience.hascodecells && python.datascience.featureenabled && !notebookEditorFocused"
+ },
+ {
+ "command": "python.datascience.selectCell",
+ "key": "ctrl+alt+shift+\\",
+ "when": "editorTextFocus && python.datascience.hascodecells && python.datascience.featureenabled && !notebookEditorFocused"
  }
  ],
  "commands": [