Theming: introduce border colors for individual components to override contrastBorder

[pahen]

I wanna add a border to the right side of the sidebar. The only way to do it now that I know of is to use contrastBorder but that's adding a lot of other borders too which I don't want. Is it possible to do it in another way? And can you add support for it if it's not possible? Needed for ayu-theme/vscode-ayu#7

[bpasero]

There are still many borders that are not configurable on their own but coming in via contrastBorder. More examples include: status bar border, activity bar border, tabs container border.

[bpasero]

I added specific border colors to override contrastBorder:

• editorGroupHeader.tabsBorder
• statusBar.border
• activityBar.border
• sideBar.border