use line_continuation instead of token literal

grammar.js

@@ -1,7 +1,7 @@
 module.exports = grammar({
   name: "dockerfile",
 
-  extras: ($) => [/\s+/, "\\\n"],
+  extras: ($) => [/\s+/, $.line_continuation],
 
   rules: {
     source_file: ($) => repeat(seq(choice($._instruction, $.comment), "\n")),

src/grammar.json

@@ -1502,8 +1502,8 @@
       "value": "\\s+"
     },
     {
-      "type": "STRING",
-      "value": "\\\n"
+      "type": "SYMBOL",
+      "name": "line_continuation"
     }
   ],
   "conflicts": [],