Add detection of generated IntelliJ files (#5617)

* Add detection of generated IntelliJ files

* Fix detection regex

Co-authored-by: John Gardner <gardnerjohng@gmail.com>

* Make documentation consistent

Co-authored-by: John Gardner <gardnerjohng@gmail.com>

Co-authored-by: John Gardner <gardnerjohng@gmail.com>
Co-authored-by: Colin Seymour <colin@github.com>

lib/linguist/generated.rb

@@ -52,6 +52,7 @@ def lines
     # Return true or false
     def generated?
       xcode_file? ||
+      intellij_file? ||
       cocoapods? ||
       carthage_build? ||
       generated_graphql_relay? ||
@@ -112,6 +113,16 @@ def generated?
     def xcode_file?
       ['.nib', '.xcworkspacedata', '.xcuserstate'].include?(extname)
     end
+
+    # Internal: Is the blob an IntelliJ IDEA project file?
+    #
+    # JetBrains IDEs generate project files under an `.idea` directory
+    # that are sometimes checked into version control.
+    #
+    # Returns true or false.
+    def intellij_file?
+      !!name.match(/(?:^|\/)\.idea\//)
+    end
 
     # Internal: Is the blob part of Pods/, which contains dependencies not meant for humans in pull requests.
     #