feat(ft): add racket support (#356)

Racket follows the same conventions as (most) other Lisps.

See https://docs.racket-lang.org/style/Choosing_the_Right_Construct.html:

lua/Comment/ft.lua

@@ -99,7 +99,7 @@ local L = setmetatable({
     markdown = { M.html, M.html },
     make = { M.hash },
     mbsyncrc = { M.dbl_hash },
-    mermaid = { "%%%s" },
+    mermaid = { '%%%s' },
     meson = { M.hash },
     nextflow = { M.cxx_l, M.cxx_b },
     nim = { M.hash, '#[%s]#' },
@@ -115,6 +115,7 @@ local L = setmetatable({
     proto = { M.cxx_l, M.cxx_b },
     quarto = { M.html, M.html },
     r = { M.hash }, -- R doesn't have block comments
+    racket = { M.lisp_l, M.lisp_b },
     readline = { M.hash },
     rego = { M.hash },
     remind = { M.hash },