Update xterm and integrate new scroll bars

[Tyriar]

Fixes #211199
Fixes #216521
See xtermjs/xterm.js#5096

The change is fairly big due to the way that the viewport is dealt with is quite different and vscode had a bunch of customizations to the old way to get it to bottom align, fill horizontal space, etc.

Wins:

• Scroll bar looks the same as in the editor and we control the styling
• Overview ruler is larger and has a left border, making it easier to see
• Sticky scroll looks much better now
• Scrolling UX is the same as in the editor, I've seen this complaint a few times where the amount the terminal scrolls is quite different
• Annoying ctrl+c not scrolling to the bottom when smooth scroll is enabled is finally fixed (The size of the vertical scroll bar in the integrated terminal does not match the size of the editor scroll bars #216521)

Changes:

• This removes the 2px padding on the very bottom of the terminal in order for the scroll bar to extend all the way to the bottom of the panel. This was a workaround to help display underscores in the terminal under certain fonts when using the DOM renderer. Let's fix issues as they come as this was a hack that degraded the default (webgl) UX.

Before:

After:

[Tyriar]

Integration test failed again, weird. Surprised this PR could impact that one