Restore dictionary validation for crawled paths

[maurosoria]

Summary

• Restore Dictionary.is_valid() so --crawl can validate discovered paths before queueing them.
• Share the validation logic with wordlist backends to keep comment, empty-path, and excluded-extension filtering consistent.
• Add regression coverage for crawled path validation.

Fixes #1576.

Validation

• /tmp/dirsearch-issue1576-venv/bin/python -m unittest tests.core.test_dictionary_templates tests.core.test_wordlist_backend
• /tmp/dirsearch-issue1576-venv/bin/python -m unittest tests.connection.test_requester tests.connection.test_dns tests.core.test_scanner
• /tmp/dirsearch-issue1576-venv/bin/python testing.py
• local --crawl smoke against an HTTP server that links /home.html to /crawled.html