Deprecate theme-color, gray, color functions

Created by: MartijnCuppens

As suggested here https://github.com/twbs/bootstrap/issues/27338#issuecomment-425655377, we can deprecate the theme-color, gray, color functions. The color and gray functions are not used by Bootstrap and it's easy to remove the theme-color dependency. These functions don't really add anything to Bootstrap.

With a small changes we can make Bootstrap independent of these functions and throw a warning like we did here, so we can deprecate them in a next major release: https://github.com/twbs/bootstrap/blob/f59c2d946c276ccc247a34ccfe6f5063c847306b/scss/mixins/_text-hide.scss#L10-L12