fix: R: Conform to Emacs highlighting conventions (#759)

* fix: increase special face priority

* fix: Apply variable only when declared, not used

Author: KaiAragaki

queries/r/highlights.scm

@@ -35,10 +35,6 @@
 
 (comma) @punctuation.delimiter
 
-; Variables
-
-(identifier) @variable
-
 ; Functions
 
 (binary_operator
@@ -62,6 +58,20 @@
 (parameters (parameter name: (identifier) @variable.parameter))
 (arguments (argument name: (identifier) @variable.parameter))
 
+; Variables
+
+(binary_operator
+    lhs: (identifier) @variable
+    operator: "="
+    rhs: (_)
+)
+
+(binary_operator
+    lhs: (identifier) @variable
+    operator: "<-"
+    rhs: (_)
+)
+
 ; Namespace
 
 (namespace_operator lhs: (identifier) @namespace)