Add "showTabsFullscreen" setting to documentation

[GeekJosh]

Adds documentation for a new "showTabsFullscreen" setting to be introduced by PR: https://github.com/microsoft/terminal/pull/18171

[learn-build-service-prod[bot]]

Learn Build status updates of commit ddecc66:

:white_check_mark: Validation status: passed

File	Status	Preview URL	Details
TerminalDocs/customize-settings/appearance.md	:white_check_mark:Succeeded	View

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[nguyen-dows]

This looks great to me! I'll keep an eye on the PR and merge this once that's merged in :)