fix(themes): unable to exclude theme mixins (#12235)

Author: SisIvanova

projects/igniteui-angular/src/lib/core/styles/components/dock-manager/_dock-manager-component.scss

@@ -10,7 +10,7 @@
 ////
 
 @mixin component {
-    @include b(igc-dock-manager) {
+    @include b(igc-dockmanager) {
         // Register the component in the component registry
         $this: bem--selector-to-string(&);
         @include register-component(

projects/igniteui-angular/src/lib/core/styles/components/stepper/_stepper-component.scss

@@ -10,7 +10,7 @@
 @mixin component {
     @include b(igx-stepper) {
         $block: bem--selector-to-string(&);
-        @include register-component($block);
+        @include register-component(string.slice($block, 2, -1));
 
         @extend %stepper-display !optional;