Improve layout and resize events in terminal

[Tyriar]

The hope is that this fixes a bunch of layout and rendering-related bugs by improving the number of columns and rows of the terminal and ensuring layout events are correctly passed through.

Some big things:

• bottom: 2px was only applied to .active terminals, so the height ended up sometimes coming through as NaN because bottom was set to auto
• Within terminalGroup.ts we now always layout the terminal instances after adding a split, this will trigger 3 layouts: the first to add the split pane (disableLayout), the second to resize it using the sizing strategy (disableLayout) and then a final one which will actually resize the terminal.
• Prevented race condition in _attachToElement by moving the xterm promise await after the elements are set on the instance

Fixes #120004
Fixes #120810
Fixes #108274

[MohamedAliRashad]

@Tyriar
When can we expect a tested release for this problem ?

[Tyriar]

@MohamedAliRashad Insiders will probably be out today or tomorrow including this.