fix(VBtn): add missing sass variable for text-transform

fixes #8619

Author: johnleider

packages/vuetify/src/components/VBtn/VBtn.sass

@@ -50,7 +50,7 @@
   position: relative
   text-decoration: none
   text-indent: $btn-letter-spacing
-  text-transform: uppercase
+  text-transform: $btn-text-transform
   transition-duration: $btn-transition-duration
   transition-property: box-shadow, transform, opacity
   transition-timing-function: $btn-transition-fn

packages/vuetify/src/components/VBtn/_variables.scss

@@ -11,6 +11,7 @@ $btn-icon-padding: 12px !default;
 $btn-letter-spacing: .0892857143em !default;
 $btn-outline-border-width: thin !default;
 $btn-rounded-border-radius: 28px !default;
+$btn-text-transform: uppercase !default;
 $btn-transition-duration: 0.28s !default;
 $btn-transition-fn: map-get($transition, 'fast-out-slow-in') !default;
 $btn-transition: opacity 0.2s map-get($transition, 'ease-in-out') !default;