feat: add external-link lua filter

Author: jooyoungseo

_extensions/external-link.lua

@@ -0,0 +1,6 @@
+function Link(el)
+  if el.target:match('^https?://') then
+    el.attributes.target = '_blank'
+  end
+  return el
+end

braille-display.qmd

@@ -1,6 +1,8 @@
 ---
 title: "How to Request an NLS eReader Braille Display?"
 format: html
+filters:
+  - external-link.lua
 ---
 
 ## Purpose of this Page