fix(stepper): selector assuming that there will always be a dir attribute (#13751)

One of the selectors in the step header assumes that there will be at least one parent with a `dir="ltr"` or `dir="rtl"`. These changes rework it to have LTR as the base.

Fixes #13741.

Author: crisbeto

src/lib/stepper/stepper.scss

@@ -76,18 +76,23 @@ $mat-stepper-line-gap: 8px !default;
     height: auto;
     padding: $mat-stepper-side-gap;
 
-    [dir='ltr'] &:not(:last-child)::after,
+    &:not(:last-child)::after,
     [dir='rtl'] &:not(:first-child)::after {
       @extend %mat-header-horizontal-line-label-position-bottom;
       right: 0;
     }
 
-    [dir='ltr'] &:not(:first-child)::before,
+    &:not(:first-child)::before,
     [dir='rtl'] &:not(:last-child)::before {
       @extend %mat-header-horizontal-line-label-position-bottom;
       left: 0;
     }
 
+    [dir='rtl'] &:last-child::before,
+    [dir='rtl'] &:first-child::after {
+      display: none;
+    }
+
     & .mat-step-icon,
     & .mat-step-icon-not-touched {
       // Cleans margin both for ltr and rtl direction