Update examples to use bslib

[skaltman]

This PR updates runExample() to use {bslib}-powered examples. These new examples are located under inst/shiny instead of inst/examples.

To run the old (legacy) examples, set options(shiny.legacy.examples = TRUE)

[CLAassistant]

All committers have signed the CLA.

[cpsievert]

Looking good to so far! I just changed file locations around a bit for better backwards-compatibility reasons.

I also suggested a couple small tweaks, and left them unresolved in case you need to make updates elsewhere

[gadenbuie]

Looks great thanks everyone!

I'd love to see us use consistent and modern code style throughout. I certainly have opinions and am happy to restyle the new examples in a follow up.

In particular, I'd like to follow these rules:

1. All arguments to a function are either on the same line or each argument is on a new line.
2. Closing braces and parens have the same level of indentation as line containing the opening function call.

# Before
      textInput(inputId = "caption",
                label = "Caption:",
                value = "Data Summary"),

      # Input: Selector for choosing dataset ----
      selectInput(inputId = "dataset",
                  label = "Choose a dataset:",
                  choices = c("rock", "pressure", "cars")),

# After
      textInput(
        inputId = "caption",
        label = "Caption:",
        value = "Data Summary"
      ),

      # Input: Selector for choosing dataset ----
      selectInput(
        inputId = "dataset",
        label = "Choose a dataset:",
        choices = c("rock", "pressure", "cars")
      ),

Importantly, this code style matches Python's black formatting rules so it's easier for us to be consistent across languages and projects if we follow it.

[gadenbuie]

@karangattu started working on extending runExample() to other packages in #3917.

That said, while we're here, I think we should do a very light-weight version of that work and add package = "shiny" to runExample()'s parameters.

I'll take that in a follow-up PR.