LibWeb: Align default “th” and “td” roles with HTML-AAM spec and tests

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This change aligns the default roles for th and td elements with the requirements in the HTML-AAM spec, and with the corresponding WPT tests at https://wpt.fyi/results/html-aam/table-roles.html, and with the behavior in other engines.

Otherwise, without this change, the default role values computed for th and td elements in some cases don’t match the behavior in other engines, and don’t match the expected results for the corresponding WPT tests.

This change gets us passing 100% of the subtests in the table-roles.html test at https://wpt.fyi/results/html-aam?product=ladybird