feat(SubmitBar): add button-color prop (#6757)

src/submit-bar/test/__snapshots__/index.spec.js.snap

@@ -1,5 +1,13 @@
 // Jest Snapshot v1, https://goo.gl/fbAQLP
 
+exports[`button-color prop 1`] = `
+<div class="van-submit-bar">
+  <div class="van-submit-bar__bar"><button class="van-button van-button--danger van-button--normal van-button--round van-submit-bar__button van-submit-bar__button--danger" style="color: rgb(255, 255, 255); background: red; border-color: red;">
+      <div class="van-button__content"></div>
+    </button></div>
+</div>
+`;
+
 exports[`decimal-length prop 1`] = `
 <div class="van-submit-bar">
   <div class="van-submit-bar__bar">

src/submit-bar/test/index.spec.js

@@ -108,3 +108,14 @@ test('disable safe-area-inset-bottom prop', () => {
   });
   expect(wrapper).toMatchSnapshot();
 });
+
+test('button-color prop', () => {
+  const wrapper = mount(SubmitBar, {
+    context: {
+      props: {
+        buttonColor: 'red',
+      },
+    },
+  });
+  expect(wrapper).toMatchSnapshot();
+});