Auto-install racket-mode if needed

CHANGELOG.md

@@ -6,6 +6,7 @@
 
 * Add a Racket module.
 * Add a Lua module.
+* Auto-install `racket-mode` if needed.
 
 ### Changes
 

core/prelude-packages.el

@@ -182,6 +182,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.pyi\\'" cython-mode cython-mode)
     ("\\.pyx\\'" cython-mode cython-mode)
     ("PKGBUILD\\'" pkgbuild-mode pkgbuild-mode)
+    ("\\.rkt\\'" racket-mode racket-mode)
     ("\\.rs\\'" rust-mode rust-mode)
     ("\\.sass\\'" sass-mode sass-mode)
     ("\\.scala\\'" scala-mode scala-mode)