Add trailingSlash option

[Rich-Harris]

Closes #733 (though I'll open a new issue for the "smart" option).

This adds trailingSlash: 'never' | 'always' | 'ignore', which keeps the existing default behaviour but also makes it easy to build sites that work with static hosting providers that expect a trailing slash for index.html pages, and provides an escape hatch for anyone that needs more complex behaviour.

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