Split up replace-border-declarations migration

[chloerice]

Right now, the replace-border-declarations migration encompasses both length migration (px/rem => tokens) and legacy Sass migration (border-radius(large) => token). Using the conventions we aligned on in this discussion, let's split up this migration into smaller migrations scoped to Sass or static styles:

• sass-replace-border
• sass-replace-border-radius
• sass-replace-border-width
• styles-tokenize-shape