Add `nav_panel()` alias for `nav()` and `navset_*()` aliases for `navs_*()`

[cpsievert]

And promote use of nav_panel() over nav(). The former is more consistent with tabPanel() and the upcoming accordion_panel().

[cpsievert]

At the same time, let's consider renaming page_navbar()'s fillable to fillable_panel.

[gadenbuie]

Linked PR: #546

[cpsievert]

Closed by #546

[github-actions]

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