Add `min_height`/`max_height` args to more `fillable` situations

[cpsievert]

Probably most needed right now by layout_sidebar(fillable = TRUE), but perhaps we should have layout_sidebar_fillable(gap, min_height = ...)?

I've also been thinking, instead of having *_fillable() everywhere, we should have fillable_options(gap, min_height, ...)/fill_options(min_height, max_height, etc) and have fill/fillable argument understand those (via htmltools::bindFillRole()

[cpsievert]

#536 should hopefully render this irrelevant

[github-actions]

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