add justfile filename to Makefile lexer (#852)

Author: silverwind

lexers/embedded/makefile.xml

@@ -12,6 +12,9 @@
     <filename>Makefile.*</filename>
     <filename>GNUmakefile</filename>
     <filename>BSDmakefile</filename>
+    <filename>Justfile</filename>
+    <filename>justfile</filename>
+    <filename>.justfile</filename>
     <mime_type>text/x-makefile</mime_type>
     <ensure_nl>true</ensure_nl>
   </config>