✨ 实现 Post.content 相关扩展协议 (#553)

* ✨ `post` 新增 `get_content()` 方法

* 🚨 make linter happy

* 💄 auto fix by pre-commit hooks

* 🐛 fix: 调整函数使用

* 💄 auto fix by pre-commit hooks

* ✨ 转用函数处理文本

* 💄 auto fix by pre-commit hooks

* 🔨 使用`Dict`存储`content_handlers`

* 💄 auto fix by pre-commit hooks

* 🎨 fix

* :arts: 简化函数使用

* 🐛 移除`Theme`的过时参数

* 🗑️ 复用 `self.plain_content`

* 💄 auto fix by pre-commit hooks

* ✨ 注册式装饰器写法

* 💄 auto fix by pre-commit hooks

* 🐛 fix

* 💄 auto fix by pre-commit hooks

* :feat: 通用纯文本处理函数

* 💄 auto fix by pre-commit hooks

* :downgrade: 复用`==`处理标题

* 🎨 简化（？）写法

* ✅ 测试修复

* ♻️ via ContentSupport extensions

* 🐛 fix test

* 💄 auto fix by pre-commit hooks

* 🐛 for clean text

* 🐛 fix

* 💄 auto fix by pre-commit hooks

* fix: for xml

* 💄 auto fix by pre-commit hooks

* chore: art

* 💄 auto fix by pre-commit hooks

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

tests/theme/test_themes.py

@@ -65,7 +65,7 @@ def mock_platform(app: App):
         async def parse(self, raw_post: "RawPost") -> "Post":
             return Post(
                 self,
-                raw_post["text"],
+                content=raw_post["text"],
                 url="http://t.tt/" + str(self.get_id(raw_post)),
                 nickname="Mock",
             )
@@ -88,7 +88,7 @@ def mock_post(app: App, mock_platform):
 
     return Post(
         m := mock_platform(ProcessContext(DefaultClientManager())),
-        "text",
+        content="text",
         title="title",
         images=["http://t.tt/1.jpg"],
         timestamp=1234567890,
@@ -98,7 +98,7 @@ def mock_post(app: App, mock_platform):
         description="description",
         repost=Post(
             m,
-            "repost",
+            content="repost",
             title="repost-title",
             images=["http://t.tt/2.jpg"],
             timestamp=1234567891,