feat: use the duckduckgo search

[treeleaves30760]

This is an update for #80.
If the firecrawl return empty in any situation, we will use the duckduckgo as a backup option.

[Dariton4000]

The changes seem well-structured and logical.

[dzhng]

thanks for this! mind that I don't merge this PR and just leave it open as a reference implementation? I want the repo to be minimal so people can easily fork / modify it

[treeleaves30760]

This change can be a feature since the firecrawl is so expensive on search, and many people suffer from it.
Also, the change is minimal and well-structured enough to maintain and modify for people.

[firefoxzheng]

Great! This feature can be easily integrated as an option to the default google search.

[serhii-nakon]

Support it!