build: don't allow trailing whitespace in CSS files (#13014)

Similar to TS files, doesn't allow trailing whitespace in CSS.

src/demo-app/screen-type/screen-type-demo.scss

@@ -10,7 +10,7 @@
   .mat-figure {
     flex-direction: column;
   }
-  
+
   &.active {
     background: rgba(0, 0, 0, 0.12);
   }

src/material-examples/expansion-steps/expansion-steps-example.css

@@ -1,4 +1,4 @@
-.example-headers-align .mat-expansion-panel-header-title, 
+.example-headers-align .mat-expansion-panel-header-title,
 .example-headers-align .mat-expansion-panel-header-description {
   flex-basis: 0;
 }

stylelint-config.json

@@ -83,6 +83,7 @@
     "selector-type-case": "lower",
     "selector-max-id": 0,
     "no-missing-end-of-source-newline": true,
+    "no-eol-whitespace": true,
     "max-line-length": 100
   }
 }