Escape newline characters in conditionalPanel expression

[wch]

This fixes #1818.

Test app:

shinyApp(
  fluidPage(
    selectInput("select", "Select",
      c(First = "first", Second = "second")
    ),
    conditionalPanel(
      condition = "input.select ==
                   'second'",
      "This should be displayed only when 'Second' is selected."
    )
  ),
  function(input, output) { }
)

[jcheng5]

@wch Can you imagine any of these other escape sequences being needed to avoid these problems? https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#Using_special_characters_in_strings

[wch]

@jcheng5 I think there needs to be a couple more characters added to the escaping, namely \r and \b. Here's an example:

# This string contains the weird characters listed at
# https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Grammar_and_types#Using_special_characters_in_strings
# including one Unicode character.
var expr =  '\0\b\f\n\t\r\v\'"\\\u5434';

var expr_escaped = expr
    .replace(/[\\"']/g, '\\$&')
    .replace(/\u0000/g, '\\0')
    .replace(/\n/g, '\\n')
    .replace(/\r/g, '\\r')
    // \b has a special meaning; need [\b] to match backspace char.
    .replace(/[\b]/g, '\\b');

eval(`"${expr_escaped}"`);

Without the added substitutions for \r and \b, the eval() gives an error. I can implement this if it looks good to you.

[wch]

I've updated it in e74592a.