Refactor configuration for highlight group appearance

Easymotion aside, the plugin currently emulates 4 vim highlight groups, Search, IncSearch, Substitute, and HighlightedYankRegion, each configured via separate background and text color keys.
Since they're all implemented as decorations, we can feasibly style much more than just borders: font weight, slant, and even overview ruler color can all be customized (In fact, most of this is already possible with simple css property injections: "vim.searchMatchTextColor": "#fff; border: 1px solid white; font-weight: bold; font-style: italic;").

Rather than add more keys to control these things, I think it might be worth deprecating the old vim.xxxColor/vim.xxxTextColor keys in favor of a new vim.xxxHighlight option, the value of which would be some subset of properties from DecorationRenderOptions:

type ConfigurableHighlight = {
  backgroundColor?: string;
  color?: string;
  borderColor?: string;
  fontStyle?: string;
  fontWeight?: string;
  textDecoration?: string;
  overviewRulerColor?: string;
};

The upside would be a more consistent interface offering a degree of customization similar to vim's highlight groups.
Of course, the downsides are an API change, and potentially more complexity. Compared to what we have now, however, I think this has the potential to be much more direct, and ultimately less confusing.

Originally posted by @adrsm108 in https://github.com/VSCodeVim/Vim/issues/7364#issuecomment-1037034557

No files or tests are named. Start by locating the existing vim.xxxColor and vim.xxxTextColor configuration and the decoration setup, then compare the supported properties with VS Code's DecorationRenderOptions. Done would require an agreed migration from the old keys to a configurable xxxHighlight option, including deprecation handling and coverage for the affected highlight groups.