CSS helpers modifiers via '-' not '--' separator

[aluarius]

can we use one dash instead of two:

          "xs": "`.gutter-1` - Extra small gutter between cells",

Originally posted by @RVitaly1978 in #2189 (comment)

[m0ksem]

@aluarius, let's mark this kind of issues with "breaking changes" label and 1.5.0 milestone, so we will be able to make a migration guide easily.

[aluarius]

Closed via #2341