make F13 to F24 keys available in SDL2 frontend

make `F13` to `F24` keys available in SDL2 frontend.  i don't see any reason not to add these keys since SLD2 supports them just fine.  some people (like me) put these keys on their mechanical keyboards, and they're great because most OSes don't map them to anything, so they're free for users

frontends/sdl2/keyboard.lisp

@@ -40,7 +40,19 @@
     (,sdl2-ffi:+sdlk-f9+ "F9" nil)
     (,sdl2-ffi:+sdlk-f10+ "F10" nil)
     (,sdl2-ffi:+sdlk-f11+ "F11" nil)
-    (,sdl2-ffi:+sdlk-f12+ "F12" nil)))
+    (,sdl2-ffi:+sdlk-f12+ "F12" nil)
+    (,sdl2-ffi:+sdlk-f13+ "F13" nil)
+    (,sdl2-ffi:+sdlk-f14+ "F14" nil)
+    (,sdl2-ffi:+sdlk-f15+ "F15" nil)
+    (,sdl2-ffi:+sdlk-f16+ "F16" nil)
+    (,sdl2-ffi:+sdlk-f17+ "F17" nil)
+    (,sdl2-ffi:+sdlk-f18+ "F18" nil)
+    (,sdl2-ffi:+sdlk-f19+ "F19" nil)
+    (,sdl2-ffi:+sdlk-f20+ "F20" nil)
+    (,sdl2-ffi:+sdlk-f21+ "F21" nil)
+    (,sdl2-ffi:+sdlk-f22+ "F22" nil)
+    (,sdl2-ffi:+sdlk-f23+ "F23" nil)
+    (,sdl2-ffi:+sdlk-f24+ "F24" nil)))
 
 (defun convert-to-sym (code)
   (let ((keyinfo (assoc code *code-name-table*)))

src/key.lisp

@@ -1,7 +1,7 @@
 (in-package :lem-core)
 
 (defvar *named-key-syms*
-  '("Backspace" "Insert" "Delete" "Down" "End" "Escape" "F0" "F1" "F10" "F11" "F12" "F2" "F3" "F4" "F5" "F6" "F7" "F8" "F9"
+  '("Backspace" "Insert" "Delete" "Down" "End" "Escape" "F0" "F1" "F10" "F11" "F12" "F13" "F14" "F15" "F16" "F17" "F18" "F19" "F2" "F20" "F21" "F22" "F23" "F24" "F3" "F4" "F5" "F6" "F7" "F8" "F9"
     "Home" "Left" "NopKey" "PageDown" "PageUp" "Return" "Right" "Space" "Tab" "Up"))
 
 (defun named-key-sym-p (key-sym)