Allow update input labels with HTML

[JohnCoene]

This is a suggestion to fix #3995.

1. We use .html() client-side (instead of .text())
2. R-side, we check if label is NULL if not we convert to character

[JohnCoene]

Note: I only include srcts and R file, I did not commit the generated JavaScript, let me know if I should.

[cpsievert]

Thanks @JohnCoene, would you mind looking into the test failures (and fixing them)?

[JohnCoene]

@cpsievert I believe it's good now.

[cpsievert]

I think this check might also now need to check for empty string? I noticed that this PR will break this "clear label" feature:

library(shiny)

ui <- fluidPage(
  checkboxInput("id", label = "foo")
)

server <- function(input, output, session) {
  updateCheckboxInput(inputId = "id", label = character(0))
}

shinyApp(ui, server)

[JohnCoene]

Ah good catch. Do we still need Array.isArray check? I'm not sure what's for to be honest; could we/can we pass a vector of length greater than 1 to label?

Asking because I wonder if it can't be simplified to just labelContent.html.length == 0 since "".length == 0 is true

[cpsievert]

@JohnCoene I noticed #3996 (comment) when having a look into modifying one of our testing apps to account for this feature. I've got a start on that here rstudio/shinycoreci#262 and it'd be awesome if you wanted to review/fix/improve that test app.

[JohnCoene]

Is there anything else I c an do on this PR?

[cpsievert]

Thanks @JohnCoene, please add a NEWS item, then this should be good to merge.

That said, it'd also be great to update our 158-input-labels test to have test coverage of this change (a minimal, still failing, proposal is in rstudio/shinycoreci#262). If you're feeling up to the task, you could also take that PR over the finish line. To run the relevant test locally:

cd shinycoreci/inst/apps/158-input-labels
R -e "shiny::runTests()"

Screenshot differences are to be expected -- as long as the screenshot changes look good locally and there are no other failures than we can merge that PR as well