bug#64792: js-mode doesn't recognize LS and PS line endings, leading to incorrect syntax highlighting

[Clément Pit-Claudel]

Hi all,

The following code prints "Hello," and "world!" in JavaScript:

//
console.debug("Hello,");
//
console.debug("world!");

There is a LINE SEPARATOR after the first //, and a PARAGRAPH SEPARATOR after 
the second //.  Both of these are valid line terminators in ECMAScript 
(https://262.ecma-international.org/10.0/#table-33), but since js-mode does not 
recognize them, it displays the whole buffer in comment face (the only 
indication that something fishy is going on is a thin box around each 
character).

Other editors handle this in various ways.  gedit treats LS and PS as line 
breaks, but still displays the first call to console.debug in comment face.  
vscode displays both LS and PS as question marks but warns about them and 
highlights the code correctly.

Is there an easy way to make Emacs treat LS and PS as newlines in js-mode?  
Ideally it would display the calls to console.debug on their own separate 
lines, like this:

//[LS]
console.debug("Hello,");
//[PS]
console.debug("world!");

… but it would already be nice to fix the syntax highlighting to not use the 
comment font.  For that these characters could be marked as comment enders.  
There may be additional work to do to check uses of \n of "^" and "$" in 
regular expressions, too.

Clément.