Add systemd lexer (#1568)

This commit adds a lexer for systemd unit files. These unit files can
have multiple extensions but this commit only supports the `*.service`
file glob at this stage.

Co-authored-by: Michael Camilleri <mike@inqk.net>

lib/rouge/demos/systemd

@@ -0,0 +1,4 @@
+[Unit]
+Description=Snap Daemon
+Requires=snapd.socket
+OnFailure=snapd.failure.service

lib/rouge/lexers/systemd.rb

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+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+module Rouge
+  module Lexers
+    class SystemD < RegexLexer
+      tag 'systemd'
+      aliases 'unit-file'
+      filenames '*.service'
+      mimetypes 'text/x-systemd-unit'
+      desc 'A lexer for systemd unit files'
+
+      state :root do
+        rule %r/\s+/, Text
+        rule %r/[;#].*/, Comment
+        rule %r/\[.*?\]$/, Keyword
+        rule %r/(.*?)(=)(.*)(\\\n)/ do
+          groups Name::Tag, Punctuation, Text, Str::Escape
+          push :continuation
+        end
+        rule %r/(.*?)(=)(.*)/ do
+          groups Name::Tag, Punctuation, Text
+        end
+      end
+
+      state :continuation do
+        rule %r/(.*?)(\\\n)/ do
+          groups Text, Str::Escape
+        end
+        rule %r/(.*)'/, Text, :pop!
+      end
+    end
+  end
+end

spec/lexers/systemd_spec.rb

@@ -0,0 +1,18 @@
+# -*- coding: utf-8 -*- #
+# frozen_string_literal: true
+
+describe Rouge::Lexers::SystemD do
+  let(:subject) { Rouge::Lexers::SystemD::new }
+
+  describe 'guessing' do
+    include Support::Guessing
+
+    it 'guesses by filename' do
+      assert_guess :filename => 'unit.service'
+    end
+
+    it 'guesses by mimetype' do
+      assert_guess :mimetype => 'text/x-systemd-unit'
+    end
+  end
+end

spec/visual/samples/systemd

@@ -0,0 +1,23 @@
+[Unit]
+Description=Snap Daemon
+Requires=snapd.socket
+OnFailure=snapd.failure.service
+# This is handled by snapd
+# X-Snapd-Snap: do-not-start
+
+[Service]
+# Disabled because it breaks lxd
+# (https://bugs.launchpad.net/snapd/+bug/1709536)
+#Nice=-5
+OOMScoreAdjust=-900
+ExecStart=/usr/lib/snapd/snapd
+EnvironmentFile=-/etc/environment
+Restart=always
+WatchdogSec=5m
+Type=notify
+SuccessExitStatus=42
+RestartPreventExitStatus=42
+KillMode=process
+
+[Install]
+WantedBy=multi-user.target