fix(themes): replace implode function with selector-append

The implode function didn't play nicely with the Dart Sass compiler. Use
the built-in selector-append function in its place.

Author: simeonoff

projects/igniteui-angular/src/lib/core/styles/base/utilities/_bem.scss

@@ -236,7 +236,7 @@ $bem--sep-mod-val: if(variable-exists(bem--sep-mod-val), $bem--sep-mod-val, '-')
                     @content;
                 }
             } @else {
-                #{implode($mx)} {
+                #{selector-append($mx...)} {
                     @content;
                 }
             }
@@ -253,7 +253,7 @@ $bem--sep-mod-val: if(variable-exists(bem--sep-mod-val), $bem--sep-mod-val, '-')
                     @content;
                 }
             } @else {
-                #{$first} #{implode($mx)} {
+                #{$first} #{selector-append($mx...)} {
                     @content;
                 }
             }
@@ -307,7 +307,7 @@ $bem--sep-mod-val: if(variable-exists(bem--sep-mod-val), $bem--sep-mod-val, '-')
         $mod-classes: append($mod-classes, #{bem-selector($block: $this, $m: $mod)})
     }
     @at-root {
-        #{implode($mod-classes)} {
+        #{selector-append($mod-classes...)} {
             @content;
         }
     }

projects/igniteui-angular/src/lib/core/styles/base/utilities/_functions.scss

@@ -326,27 +326,6 @@
     );
 }
 
-/// Returns a string from the elements of a list.
-/// Works recursively so the elements can be lists themselves.
-/// @param {List} $list [null] - A list of elements to be turned into a string.
-/// @param {String} $glue [''] - A string symbol to join the elements by.
-@function implode($list, $glue: '') {
-    $result: null;
-    @for $i from 1 through length($list) {
-        $e: nth($list, $i);
-        @if type-of($e) == list {
-            $result: $result#{implode($e, $glue)};
-        } @else {
-            $result: if(
-                $i != length($list),
-                $result#{$e}#{$glue},
-                $result#{$e}
-            );
-        }
-    }
-    @return $result;
-}
-
 /// Returns a CSS property value. It could return either css '--var' or
 /// a plain string value based on theme configuration.
 /// @access public