Preview file from explorer is not tracked as jump

[yaegaki]

Describe the bug
Preview file from explorer is not tracked as jump
so i can not back to previous file.

To Reproduce
Steps to reproduce the behavior:

1. set workbench.editor.enablePreview to true.
2. preview file from explorer.
3. preview other file from explorer.
4. type <C-o>
5. can not back to first preview file.

Expected behavior
Preview file is tracked as jump.

Screenshots

• preview lexter.text.ts -> parser.test.ts -> scanner.test.ts.
• type <C-o>
• expect parser.test.ts was opend, but not

Environment (please complete the following information):

• Extension Version: 1.0.8
• OS Version: Windows_NT x64 10.0.17134
• VSCode version: 1.31.1

Additional context

vscode.workspace.onDidCloseTextDocument event is fired before vscode.window.onDidChangeActiveTextEditor event.
so old ModeHandler has already been deleted from ModeHandlerMap when fire vscode.window.onDidChangeActiveTextEditor.

Before this commit(61cba7a), Deletion was delayed because await was used in vscode.workspace.onDidCloseTextDocument handler.