fix(material-experimental/theming): Update M3 palettes (#28109)

Last time I checked, the M3 theme generator generated the same neutral
palette regardless of the primary color used. That didn't really make
much sense, because it caused the neutral colors to always have a
purplish tint. It seems like maybe a bug that was fixed, because now it
generates different neutral colors depending on the primary.

I've updated our pre-defined palettes to capture these different
neutrals. I created a script to convert the MDC theme builder exports to
our palette CSS format. I'm not including it as part of this commit, but
just FYI that it'll be cheap to add to / update our predefined palettes
in the future. If we want the script checked in I can clean it up and
make a separate PR for it.

(cherry picked from commit 1735032)

Author: mmalerba

src/material-experimental/theming/_definition.scss

@@ -38,7 +38,7 @@ $theme-version: 1;
   }
 
   $type: map.get($config, theme-type) or light;
-  $primary: map.get($config, primary) or m3-palettes.$indigo-palette;
+  $primary: map.get($config, primary) or m3-palettes.$purple-palette;
   $secondary: map.get($config, secondary) or $primary;
   $tertiary: map.get($config, tertiary) or $secondary;
 
@@ -47,15 +47,15 @@ $theme-version: 1;
       theme-version: $theme-version,
       theme-type: $type,
       palettes: (
-        primary: map.remove($primary, neutral-variant),
-        secondary: map.remove($secondary, neutral-variant),
-        tertiary: map.remove($tertiary, neutral-variant),
-        neutral: m3-palettes.$neutral-palette,
+        primary: map.remove($primary, neutral, neutral-variant),
+        secondary: map.remove($secondary, neutral, neutral-variant),
+        tertiary: map.remove($tertiary, neutral, neutral-variant),
+        neutral: map.get($primary, neutral),
         neutral-variant: map.get($primary, neutral-variant),
-        error: m3-palettes.$red-palette
+        error: map.remove(m3-palettes.$red-palette, neutral, neutral-variant),
       ),
-      color-tokens: m3-tokens.generate-color-tokens($type, $primary, $secondary, $tertiary,
-        m3-palettes.$neutral-palette, m3-palettes.$red-palette)
+      color-tokens: m3-tokens.generate-color-tokens(
+          $type, $primary, $secondary, $tertiary, m3-palettes.$red-palette)
     )
   );
 }