feat: complete dart code for chapter_searching and chapter_sorting

[gvenusleo]

If this PR is related to coding or code translation, please fill out the checklist and paste the console outputs to the PR.

• [x] I've tested the code and ensured the outputs are the same as the outputs of reference codes.
• [x] I've checked the codes (formatting, comments, indentation, file header, etc) carefully.
• [x] The code does not rely on a particular environment or IDE and can be executed on a standard system (Win, macOS, Ubuntu).

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1 Ignored Deployment

Name	Status	Preview	Comments	Updated (UTC)
hello-algo	⬜️ Ignored (Inspect)			Jun 1, 2023 1:35am

[gvenusleo]

There are the console outputs:

• binary_search_edge.dart

  数组中最左一个元素 6 的索引 = 2
  数组中最右一个元素 6 的索引 = 6
  

• heap_sort.dart

  堆排序完成后 nums = [1, 1, 2, 3, 4, 5]
  

• selection_sort.dart

  选择排序完成后 nums = [1, 1, 2, 3, 4, 5]