message: feat(button): add loading styles
diff: font: var(--typography-button-font);
  justify-content: center;
  letter-spacing: var(--typography-button-letter);
  + min-width: var(--space-xlg);
  overflow: hidden;
  position: relative;
  text-decoration: none;
  }
  }
  +.isLoading {
  + cursor: default;
  +
  + .children,
  + .startIcon,
  + .endIcon {
  + opacity: 0;
  + }
  +}
  +
  .default {
  &.text {
  color: var(--default-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--default-color-lighten);
  opacity: 0.1;
  border-color: var(--default-color);
  color: var(--default-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--default-color-lighten);
  opacity: 0.1;
  background-color: var(--default-color);
  color: var(--default-color-contrast);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--default-color-lighten);
  opacity: 1;
  &.text {
  color: var(--primary-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--primary-color-lighten);
  opacity: 0.1;
  border-color: var(--primary-color);
  color: var(--primary-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--primary-color-lighten);
  opacity: 0.1;
  background-color: var(--primary-color);
  color: var(--primary-color-contrast);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--primary-color-lighten);
  opacity: 1;
  &.text {
  color: var(--secondary-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--secondary-color-lighten);
  opacity: 0.1;
  border-color: var(--secondary-color);
  color: var(--secondary-color);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--secondary-color-lighten);
  opacity: 0.1;
  background-color: var(--secondary-color);
  color: var(--secondary-color-contrast);
  - &:not([disabled]):hover {
  + &:not([disabled]):not(.isLoading):hover {
  &::before {
  background-color: var(--secondary-color-lighten);
  opacity: 1;
  font-size: 1em;
  }
  }
  +
  +.loading {
  + display: block;
  + position: absolute;
  + z-index: calc(var(--zindex-5) + 1);
  +}
  