refactor: splitting and reorganizing style files

packages/@core/shared/design/src/scss-bem/bem.scss

@@ -0,0 +1,34 @@
+@forward './constants.scss';
+
+@mixin b($block) {
+  $B: $namespace + '-' + $block !global;
+
+  .#{$B} {
+    @content;
+  }
+}
+
+@mixin e($name) {
+  @at-root {
+    &#{$element-separator}#{$name} {
+      @content;
+    }
+  }
+}
+
+@mixin m($name) {
+  @at-root {
+    &#{$modifier-separator}#{$name} {
+      @content;
+    }
+  }
+}
+
+// block__element.is-active {}
+@mixin is($state, $prefix: $state-prefix) {
+  @at-root {
+    &.#{$prefix}-#{$state} {
+      @content;
+    }
+  }
+}

packages/@core/shared/design/src/scss-bem/constants.scss

@@ -0,0 +1,5 @@
+$namespace: 'vben' !default;
+$common-separator: '-' !default;
+$element-separator: '__' !default;
+$modifier-separator: '--' !default;
+$state-prefix: 'is' !default;