feat: Add Dart Codes for section_iteration_and_recursion

[gvenusleo]

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

• I've tested the code and ensured the outputs are the same as the outputs of reference codes.
• I've checked the codes (formatting, comments, indentation, file header, etc) carefully.
• 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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[gvenusleo]

The terminal output of each file is as follows:

iteration.dart

for 循环的求和结果 res = 15

while 循环的求和结果 res = 15

while 循环（两次更新）的求和结果 res = 5

双层 for 循环的结果 (1, 1), (1, 2), (1, 3), (1, 4), (1, 5), (2, 1), (2, 2), (2, 3), (2, 4), (2, 5), (3, 1), (3, 2), (3, 3), (3, 4), (3, 5), (4, 1), (4, 2), (4, 3), (4, 4), (4, 5), (5, 1), (5, 2), (5, 3), (5, 4), (5, 5),

recursion.dart

递归函数的求和结果 res = 15

尾递归函数的求和结果 res = 15

斐波那契数列的第 5 项为 3

[krahets]

Thanks for the PR!