Add support for color palettes in themes

App themes have many color tokens, but most of these colors tend to be limited to much smaller sets of colors. It would be useful to have a way to refer to these color "swatches" rather than the raw color values over and over again. This makes it easier to make color adjustments while keeping the app's color palette uniform.

For example, today we must specify explicit color values for every token:

{
  "data-grid-focus-cell-selected-hover-background": "#3669FF",
  "data-grid-focus-cell-selected-hover-border": "#3669FF",
  "data-grid-focus-cell-selected-inactive-background": "#3355AA",
  "data-grid-focus-cell-selected-inactive-border": "#3355AA",
}

It would be more convenient to refer to a color by some name, allowing color palettes to remain unform and future changes to be localized to one place:

{
  "DarkBlue": "#3355AA",
  "LightBlue": "#3669FF",
  "data-grid-focus-cell-selected-hover-background": "%LightBlue",
  "data-grid-focus-cell-selected-hover-border": "%LightBlue",
  "data-grid-focus-cell-selected-inactive-background": "%DarkBlue",
  "data-grid-focus-cell-selected-inactive-border": "%DarkBlue"
}

{
  "colors": {
    "DarkBlue": "#3355AA",
    "LightBlue": "#3669FF"
  },
  "tokens": {
    "data-grid-focus-cell-selected-hover-background": "%LightBlue",
    "data-grid-focus-cell-selected-hover-border": "%LightBlue",
    "data-grid-focus-cell-selected-inactive-background": "%DarkBlue",
    "data-grid-focus-cell-selected-inactive-border": "%DarkBlue"
  }
}

Start by locating the theme token handling and configuration used by Azure Storage Explorer; the issue names no files, tests, or entry points. Compare the two palette representations in the issue and define what “done” means for resolving named colors while preserving existing raw color values.