fix: 修复uni-id相关接口使用atob在不支持的平台导致报错的Bug

src/shared/uni-id-mixin.js

+let realAtob
+
+const b64 = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/='
+const b64re = /^(?:[A-Za-z\d+/]{4})*?(?:[A-Za-z\d+/]{2}(?:==)?|[A-Za-z\d+/]{3}=?)?$/
+
+if (typeof atob !== 'function') {
+  realAtob = function (str) {
+    str = String(str).replace(/[\t\n\f\r ]+/g, '')
+    if (!b64re.test(str)) { throw new Error("Failed to execute 'atob' on 'Window': The string to be decoded is not correctly encoded.") }
+
+    // Adding the padding if missing, for semplicity
+    str += '=='.slice(2 - (str.length & 3))
+    var bitmap; var result = ''; var r1; var r2; var i = 0
+    for (; i < str.length;) {
+      bitmap = b64.indexOf(str.charAt(i++)) << 18 | b64.indexOf(str.charAt(i++)) << 12 |
+                    (r1 = b64.indexOf(str.charAt(i++))) << 6 | (r2 = b64.indexOf(str.charAt(i++)))
+
+      result += r1 === 64 ? String.fromCharCode(bitmap >> 16 & 255)
+        : r2 === 64 ? String.fromCharCode(bitmap >> 16 & 255, bitmap >> 8 & 255)
+          : String.fromCharCode(bitmap >> 16 & 255, bitmap >> 8 & 255, bitmap & 255)
+    }
+    return result
+  }
+} else {
+  // 注意atob只能在全局对象上调用，例如：`const Base64 = {atob};Base64.atob('xxxx')`是错误的用法
+  realAtob = atob
+}
+
 function b64DecodeUnicode (str) {
-  return decodeURIComponent(atob(str).split('').map(function (c) {
+  return decodeURIComponent(realAtob(str).split('').map(function (c) {
     return '%' + ('00' + c.charCodeAt(0).toString(16)).slice(-2)
   }).join(''))
 }