Trailing Space should use region.redish by default, instead of invalid

[arianf]

Trailing Space should use region.redish by default, instead of invalid.

Around Sublime build 3148, region-ish colors were added. These are scopes that get auto-generated for a color scheme (but can also be explicitly defined) that are meant to define a way for users to pick a specific color that corresponds to the color palette of the theme. TrailingSpaces should now default to

{
  "trailing_spaces_highlight_color": "region.redish"
}