improve: 美化插件树的显示

[he0119]

现在这样好看多了。

测试 # 一个测试插件
├── 复杂功能 # 测试插件复杂子插件
│   └── 二级功能 # 测试插件二级插件
└── 简单功能 # 测试插件简单子插件
帮助 # 获取插件帮助信息'

[codecov]

Codecov Report

Merging #27 (4c918dc) into main (9b40e70) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #27   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            2         2           
  Lines           66        71    +5     
=========================================
+ Hits            66        71    +5     

Impacted Files	Coverage Δ
nonebot_plugin_treehelp/data_source.py	100.00% <100.00%> (ø)

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