tartrazine/lexers/plaintext.xml

<lexer>
  <config>
    <name>plaintext</name>
    <alias>text</alias>
    <alias>plain</alias>
    <alias>no-highlight</alias>
    <filename>*.txt</filename>
    <mime_type>text/plain</mime_type>
    <priority>-1</priority>
  </config>
  <rules>
    <state name="root">
      <rule pattern=".+">
        <token type="Text"/>
      </rule>
      <rule pattern="\n">
        <token type="Text"/>
      </rule>
    </state>
  </rules>
</lexer>