Use v2 components

[kamilzyla]

Problem

Some components in Blueprint have v2 versions with changed interface, e.g. PanelStack and PanelStack2. Currently we are somewhat inconsistent with them, e.g. shiny.blueprint::Select is the React Select component, but shiny.blueprint::Select.shinyInput uses Select2.

It seems reasonable to only expose the v2 versions: we'd better avoid exposing the deprecated versions. It raises a question however: should we keep the 2 suffix in component names? It seems reasonable to drop it: having Select2 without Select will be confusing; the name probably won't have the suffix in the upcoming Blueprint v5.

Goal

1. Consistently use v2 versions whenever available - both for plain components (e.g. shiny.blueprint::Select) and their .shinyInput wrappers.
2. Clearly document:
   • We are wrapping Blueprint v4.
   • Where applicable, the components use the latest API. For example, shiny.blueprint::Select is actually the Select2 component.