Use ggplot2::get_alt_text() if available to provide better default alt text

[cpsievert]

Closes #3397 (via tidyverse/ggplot2#4482)

library(shiny)

ui <- fluidPage(
  plotOutput("p")
)

server <- function(input, output, session) {
  output$p <- renderPlot({
    ggplot(mtcars, aes(wt, cyl)) + 
      geom_point() +
      labs(alt = "Foo bar")
  })
}

shinyApp(ui, server)

[wch]

Looks good, provided that the PR on the ggplot2 side stays pretty much the same.