Update misc.ts 导出 BASE64_KEYS

[finscn]

项目中自己处理 base64 时 需要用到.
而且 BASE64_VALUES 都已经导出了, 导出 BASE64_KEYS 也是很自然的事.

虽然这玩意自己写也不复杂 但是既然 cocos 已经有了 , 就导出一下吧

引擎的意义之一不就是让开发者省事吗.

Re: #

Changelog

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
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[github-actions]

Code Size Check Report

Wechat (WASM)	Before	After	Diff
2D Empty (legacy pipeline)	1009778 bytes	1009814 bytes	⚠️ +36 bytes
2D All (legacy pipeline)	2675337 bytes	2675376 bytes	⚠️ +39 bytes
2D All (new pipeline)	2767073 bytes	2767113 bytes	⚠️ +40 bytes
(2D + 3D) All	10024043 bytes	10024082 bytes	⚠️ +39 bytes

Web (WASM + ASMJS)	Before	After	Diff
(2D + 3D) All	16843105 bytes	16843144 bytes	⚠️ +39 bytes

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -20825,8 +20825,9 @@
         export const BUILTIN_CLASSID_RE: RegExp;
         export interface Modifiable {
             getModifiableProperties(): string[];
         }
+        export const BASE64_KEYS = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/=";
         export const BASE64_VALUES: number[];
     }
     export namespace path {
         /**