feat: add ability to run code samples as tests (#252)

[assassinaj602]

Description

Adds ability to actually run code samples marked with #test tag.

What It Does

• Detects #test tag in code samples
• Runs the code as a test using dart run
• Reports pass/fail results
• New --run flag to enable testing

Example

/// ```dart#test
/// assert(2 + 2 == 4);
/// ```
int add(int a, int b) => a + b;