[themes] Allow custom variables and references in workbench color customizations section

This is a feature request to allow defining custom color variables in the workbench color customizations section. It would allow creating a small palette of colors and reusing it across the workbench color definitions, saving many repetitions and making it much easier to maintain.

This is just simple text substitution based on some naming convention like the .less @variable convention. For example, see the @PaletteXXX variables in the sample below:

"workbench.colorCustomizations": {

      // custom colors
      "@Palette000": "#FFFFFF",
      "@Palette050": "#ECEFF1",
      "@Palette100": "#CFD8DC",
      "@Palette200": "#B0BEC5",
      "@Palette300": "#90A4AE",
      "@Palette700": "#455A64",

      // input control
      "input.background": "@Palette000",
      "input.foreground": "@Palette700",
      "input.placeholderForeground": "@Palette200",
      "inputOption.activeBorder": "@Palette300",
      "input.border": "@Palette100",

      // list views
      "list.activeSelectionBackground": "@Palette200",
      "list.activeSelectionForeground": "@Palette700",
      "list.inactiveSelectionBackground": "@Palette100",
      "list.hoverBackground": "@Palette100",
      "list.dropBackground": "@Palette100",
      "list.focusBackground": "@Palette100",

[aeschli]

I’m expanding the issue to ‘variables’ and ‘references’

[memeplex]

https://github.com/microsoft/vscode/issues/73069 was closed as dup of this although I believe the intention was different: to refer to colors already defined by themes, not to variables defined by the end user. For example, if I want the base foreground color for markdown emphasis, instead of the more colorful variants many themes apply, I would like to express “for this use what this theme defines as the default foreground color” without having to hardcode colors for each theme.