Expose Viewport syncScrollArea() to Terminal and include force param

[bdrazen]

To allow explicit invocation of scroll sync.

See: microsoft/vscode#58532

[Tyriar]

I think xterm.js should be smart enough to not need to expose this, it should just work. There's a root cause somewhere, perhaps related to #1607?

[Tyriar]

I notice in vscode that scroll to bottom/top commands don't do anything to the scrollbar, that seems wrong as all they do is forward calls to xterm.js. This also appears to be the case for scroll by line/page as well.

https://github.com/Microsoft/vscode/blob/7ccd93ed5ea3e21b5cf70db7ba73e3adfdd0d0d9/src/vs/workbench/parts/terminal/electron-browser/terminalInstance.ts#L684-L706

[Tyriar]

Fixed in #1698, thanks for the help 😃