fix(material/button): Move fab unthemable tokens to theme mixin (#27580)

Though these tokens are not currently affected by the theme, in the
future they will be affected by the design system used for theming (M2
or M3)

BREAKING CHANGE:
There are new styles emitted by `mat.fab-theme` that are not
emitted by any of: `mat.fab-color`, `mat.fab-typography`,
`mat.fab-density`. If you rely on the partial mixins only and don't
call `mat.fab-theme`, you can add `mat.fab-base` to get the
missing styles.

src/material/_index.scss

@@ -88,7 +88,7 @@
 @forward './button/button-theme' as button-* show button-theme, button-color, button-typography,
   button-density;
 @forward './button/fab-theme' as fab-* show fab-color, fab-typography,
-  fab-density, fab-theme;
+  fab-density, fab-theme, fab-base;
 @forward './button/icon-button-theme' as icon-button-* show icon-button-color,
   icon-button-typography, icon-button-density, icon-button-theme;
 @forward './button-toggle/button-toggle-theme' as button-toggle-* show button-toggle-theme,

src/material/button/_fab-theme.scss

@@ -5,11 +5,22 @@
 
 @use './button-theme-private';
 @use '../core/mdc-helpers/mdc-helpers';
+@use '../core/style/sass-utils';
 @use '../core/theming/theming';
 @use '../core/tokens/m2/mdc/fab' as tokens-mdc-fab;
 @use '../core/tokens/m2/mdc/extended-fab' as tokens-mdc-extended-fab;
 @use '../core/typography/typography';
 
+@mixin base($config-or-theme) {
+  // Add default values for tokens not related to color, typography, or density.
+  @include sass-utils.current-selector-or-root() {
+    @include mdc-fab-theme.theme(tokens-mdc-fab.get-unthemable-tokens());
+    @include mdc-extended-fab-theme.theme(
+      tokens-mdc-extended-fab.get-unthemable-tokens()
+    );
+  }
+}
+
 @mixin _fab-variant($config, $foreground, $background) {
   $color-config: map.merge(
     $config,
@@ -105,6 +116,7 @@
     $density: theming.get-density-config($theme);
     $typography: theming.get-typography-config($theme);
 
+    @include base($theme);
     @if $color != null {
       @include color($color);
     }

src/material/button/fab.scss

@@ -22,17 +22,11 @@
   .mat-mdc-fab, .mat-mdc-mini-fab {
     // Add the official slots for the MDC fab.
     @include mdc-fab-theme.theme-styles($mdc-fab-token-slots);
-
-    // Add default values for tokens that aren't outputted by the theming API.
-    @include mdc-fab-theme.theme(m2-mdc-fab.get-unthemable-tokens());
   }
 
   .mat-mdc-extended-fab {
     // Add the official slots for the MDC fab.
     @include mdc-extended-fab-theme.theme-styles($mdc-extended-fab-token-slots);
-
-    // Add default values for tokens that aren't outputted by the theming API.
-    @include mdc-extended-fab-theme.theme(m2-mdc-extended-fab.get-unthemable-tokens());
   }
 }