v5: Add .fs-* utilities for font-size and rename font-weight/-style utilities (#30571)

v5: Add .fs-* utilities for font-size and rename font-weight/-style utilities (#30571)

* v5: Add .fs-* utilities for font-size

- Adds new font-sizes Sass map
- Generates six new classes for setting only font-size
- Updates docs to mention this, including a scss-docs reference

* Update font utilities

- Make .fs-* utils for font-size use RFS
- Rename .font-weight-* utils to .fw-*
- Rename .font-style-* utils to .f-*
- Update order of utilities a bit
- Update docs to match

* Update migration docs to make note of these changes

* Be more specific in font-size docs about scale

* Update font-style abbreviation to .fst

* Fix font-weight property

* Formatting & font style utility fix

* Move to Alpha 3 migration section

* Update migration guide for more details, splitting alpha 2 stuff back to the appropriate section in Migration guide

* Tweak language on example

Co-authored-by: Martijn Cuppens <martijn.cuppens@gmail.com>