fix: improve styling of 'clear_button' selectize plugin

[cpsievert]

This PR improves the styling of selectize's "clear_button" plugin option, which we intend on making the default over in posit-dev/py-shiny#2064

library(shiny)

ui <- bslib::page_fluid(
  selectizeInput(
    "foo", "Foo", choices = c("Option 1", "Option 2", "Option 3"),
    options = list(plugins = list("clear_button"))
  )
)

server <- function(input, output, session) {}

shinyApp(ui, server)

Before

After