Add runtime regex notification checks

[JeremyZeng77]

Summary

Fixes #2.

Adds runtime-configurable regex notification checks via --checks / --check-config.

• Supports JSON config files as either an array or { "checks": [...] }
• Supports DNS fields (type, info/rdata, ttl), HTTP fields (http_code, code_string, content_length), domain names, and page-content aliases (body, response_body, page_content, content)
• Captures HTTP response bodies only in memory for matching, capped at 64 KiB per request, without writing bodies into the JSON output
• Validates config fields and regex patterns before probing starts
• Documents the config shape and example usage in README.md

Verification

• git diff --cached --check before commit
• Compiled the new regex check module with MSVC directly:
  cl /nologo /std:c++17 /EHsc /I dooked\include /I dooked\json\single_include /I dooked\spdlog\include /c dooked\source\checks\regex_checks.cpp

Full CMake build was not run in this environment because cmake is not installed and the local VS DevCmd initialization fails before setting the compiler environment.

Bounty

Bounty issue: #2

If accepted and merged for the bounty, PayPal payout destination: https://paypal.me/Jeremytsangchina