feat(Dialog): add new prop theme (#6921)

* feat(dialog): add new prop button-theme & change default confirm button text color

* test(dialog): update snapshot

* refactor(dialog): rename button-theme to theme

* refactor(dialog): add theme definition

* docs(dialog): add version tip & change default confirm-button-color

src/dialog/test/__snapshots__/demo.spec.js.snap

@@ -11,6 +11,15 @@ exports[`renders demo correctly 1`] = `
       提示弹窗（无标题）
     </span></div>
     </button></div>
+  <div><button class="van-button van-button--primary van-button--normal">
+      <div class="van-button__content"><span class="van-button__text">
+      提示弹窗
+    </span></div>
+    </button> <button class="van-button van-button--primary van-button--normal">
+      <div class="van-button__content"><span class="van-button__text">
+      提示弹窗（无标题）
+    </span></div>
+    </button></div>
   <div><button class="van-button van-button--primary van-button--normal">
       <div class="van-button__content"><span class="van-button__text">
       确认弹窗