[VS Code] configure username for blame

[Ammar Bandukwala]

See #716 for context.

When the OS username is shared or doesn't match up with the commit author for some other reason, the (you) indicator behaves incorrectly.

[Evan Krause]

I think we would probably want to accomplish this by using sl config ui.username as the source of this info, instead of os.userInfo().username.

This is probably not a high priority for us, but the change might look like this:
https://github.com/facebook/sapling/blob/main/addons/vscode/extension/blame/blame.ts#L32

• make this function a method of InlineBlameProvider
• stop using getUsername() from isl-server, and instead use this.currentRepo.getConfig('ui.username')
• cache that value somewhere so we only fetch it once