Heuristic rule for TeX .cls files (#3360)

lib/linguist/heuristics.rb

@@ -110,6 +110,12 @@ module Linguist
       end
     end
 
+    disambiguate ".cls" do |data|
+      if /\\\w+{/.match(data)
+        Language["TeX"]
+      end
+    end
+
     disambiguate ".cs" do |data|
       if /![\w\s]+methodsFor: /.match(data)
         Language["Smalltalk"]

test/test_heuristics.rb

@@ -73,6 +73,15 @@ class TestHeuristcs < Minitest::Test
     })
   end
 
+  def test_cls_by_heuristics
+    assert_heuristics({
+      "TeX" => all_fixtures("TeX", "*.cls"),
+      nil => all_fixtures("Apex", "*.cls"),
+      nil => all_fixtures("OpenEdge ABL", "*.cls"),
+      nil => all_fixtures("Visual Basic", "*.cls"),
+    })
+  end
+
   def test_cs_by_heuristics
     assert_heuristics({
       "C#" => all_fixtures("C#", "*.cs"),