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>

scss/_variables.scss

@@ -425,6 +425,17 @@ $h4-font-size:                $font-size-base * 1.5 !default;
 $h5-font-size:                $font-size-base * 1.25 !default;
 $h6-font-size:                $font-size-base !default;
 
+// scss-docs-start font-sizes
+$font-sizes: (
+  1: $h1-font-size,
+  2: $h2-font-size,
+  3: $h3-font-size,
+  4: $h4-font-size,
+  5: $h5-font-size,
+  6: $h6-font-size
+) !default;
+// scss-docs-end font-sizes
+
 $headings-margin-bottom:      $spacer / 2 !default;
 $headings-font-family:        null !default;
 $headings-font-style:         null !default;