fix(material/button): ensure svg icon is centered

Fixes that SVG icons sometimes weren't centered in an icon button.

Fixes #26126.

Author: crisbeto

src/material/button/icon-button.scss

@@ -45,6 +45,15 @@
     border-radius: 50%;
   }
 
+  // Ensures that the icons are centered.
+  .mat-icon {
+    text-align: center;
+
+    svg {
+      vertical-align: baseline;
+    }
+  }
+
   // MDC adds some styles to icon buttons that conflict with some of our focus indicator styles
   // and don't actually do anything. This undoes those conflicting styles.
   &.mat-unthemed,