fix: capture backtick escaped variable

Author: randy3k

syntaxes/r.json

@@ -40,6 +40,9 @@
     },
     {
       "include": "#function-calls"
+    },
+    {
+      "match": "(?:[a-zA-Z._][\\w.]*|`[^`]+`)"
     }
   ],
   "repository": {

syntaxes/r.yaml

@@ -25,6 +25,7 @@ patterns:
   - include: "#lambda-functions"
   - include: "#builtin-functions"
   - include: "#function-calls"
+  - match: '{{var}}'
 
 repository:
   accessor:

tests/testdata/basics.R

@@ -152,3 +152,7 @@ foo::median
 my_vector |>
 #         ^^ keyword.operator.pipe.r
   foo()
+
+# issue #1
+foo(x = `#afd`)
+#        ^^^^ -comment