feat(css): Add pajamas typescale variables

This MR is part of the effort to implement Pajamas typescale in gitlab-ui. For more information, read the following issue: #373 (closed)

What does this MR do?

It adds SASS variables that represent all the possible text sizes defined in Pajamas Typescale specification. These variables will be used to implement typescale classes, utility-classes, etc. It also documents the naming convention used to declare these variables and all the possible parameters that affect text size.