Denote private nature of heading/body mixins

packages/base-styles/_mixins.scss

@@ -5,66 +5,66 @@
  * Typography
  */
 
-@mixin text-heading() {
+@mixin _text-heading() {
 	font-family: $font-family-headings;
 	font-weight: $font-weight-medium;
 }
 
-@mixin text-body() {
+@mixin _text-body() {
 	font-family: $font-family-body;
 	font-weight: $font-weight-regular;
 }
 
 @mixin heading-small() {
-	@include text-heading();
+	@include _text-heading();
 	font-size: $font-size-x-small;
 	line-height: $line-height-x-small;
 }
 
 @mixin heading-medium() {
-	@include text-heading();
+	@include _text-heading();
 	font-size: $font-size-medium;
 	line-height: $line-height-small;
 }
 
 @mixin heading-large() {
-	@include text-heading();
+	@include _text-heading();
 	font-size: $font-size-large;
 	line-height: $line-height-small;
 }
 
 @mixin heading-x-large() {
-	@include text-heading();
+	@include _text-heading();
 	font-size: $font-size-x-large;
 	line-height: $line-height-medium;
 }
 
 @mixin heading-2x-large() {
-	@include text-heading();
+	@include _text-heading();
 	font-size: $font-size-2x-large;
 	line-height: $font-line-height-2x-large;
 }
 
 @mixin body-small() {
-	@include text-body();
+	@include _text-body();
 	font-size: $font-size-small;
 	line-height: $line-height-x-small;
 }
 
 @mixin body-medium() {
-	@include text-body();
+	@include _text-body();
 	font-size: $font-size-medium;
 	line-height: $line-height-small;
 }
 
 @mixin body-large() {
-	@include text-body();
+	@include _text-body();
 	font-size: $font-size-large;
 	line-height: $line-height-medium;
 }
 
 @mixin body-x-large() {
-	@include text-body();
+	@include _text-body();
 	font-size: $font-size-x-large;
 	line-height: $line-height-x-large;
 }