Support for decorations to change cell foreground and background

[Tyriar]

Fixes #3770

This new feature works by swapping out the the background and foreground colors of the cell. This is slightly different depending on the renderer especially wrt getting the correct cache key to use which is why caching of decoration override glyphs isn't supported at all for the canvas renderer.

Notice how the text is white:

Current API shape:

export interface IDecorationOptions {
  readonly backgroundColor?: string;
  readonly foregroundColor?: string;
}

Note the API doesn't allow changing the colors currently as that's not actually a requirement for the search addon.

Remaining work:

• Webgl renderer tests
• Expose an interface on IDecorationService to get decorations at a cell to reduce code duplication
• Maintain a sorted list of decorations (by line) in IDecorationService and utilize it to go from O(n) to O(log n) when iterating over decorations of a line
• Review TODOs/clean up

Deferred from the PR:

• Alpha support - can look into it if we need it
• Allow the webgl renderer to draw decorations on top of the selection - this is required in order to get consistency in how VS Code's editor looks and works. My idea here is to bring selection rendering into the main glyph rendering part similar to how overrides work and then add another decoration option for whether it's above or below the selection. bg/fg decorations should be able to render on top of the selection #3778

Other issues fixed:

• Minimum contrast ratio was using the wrong channel when determining luminance which would throw the resulting color off.
• Decorations are now cleared on Terminal.reset()