feat: 支持输出插件树

[he0119]

通过 /help tree 输出插件树。

帮助 # 获取插件帮助信息
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

[codecov]

Codecov Report

Merging #15 (fc8a0ea) into main (08b3555) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           25        39   +14     
=========================================
+ Hits            25        39   +14     

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

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