Change default of `comments.openView` to `firstFile`

[alexr00]

There's been lots of feedback that the current value file is too aggressive:
microsoft/vscode-pull-request-github#3459
microsoft/vscode-pull-request-github#3452

[alexr00]

The firstFile value will cause the comments view to open the first time a file with comments is active per session (a reload will count as a new session).

[valsworthen]

Thank you a lot for this feature! Will it be available as of tomorrow in insiders downloads?

[alexr00]

Yes, the new default setting value will be available in the next insiders.