collect: option for extensionless URLs

[fulldecent]

In this configuration set for collect:

https://github.com/GoogleChrome/lighthouse-ci/blob/main/docs/configuration.md#collect

when working with static folder, please add an option (on by default?) so that a hyperlink to /file will properly load if /file.html exists.

Some people remove the .html part of their URLs to make them look cool and the collect script could please support that.

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Test case:

https://github.com/fulldecent/github-pages-template/tree/b999633b792def8e61b1284ebdad19f2ab8f27cc/source/experiment-template