Make plugin editions use external core (#8952)

* Make some editions use external core

* Revert tw5.com-docs use external js

* Revert "Revert tw5.com-docs use external js"

This reverts commit 93f36cb0af.

editions/markdowndemo/tiddlers/$__coreURL.tid

@@ -0,0 +1,4 @@
+title: $:/coreURL
+tags: $:/tags/Global
+
+\function coreURL() [[../../../tiddlywikicore-$(version)$.js]substitute[]]

editions/markdowndemo/tiddlywiki.info

@@ -13,6 +13,9 @@
 	],
 	"build": {
 		"index": [
-			"--rendertiddler","$:/core/save/all","markdowndemo.html","text/plain"]
+			"--rendertiddler","$:/core/save/all","markdowndemo.html","text/plain"],
+		"external": [
+			"--render","$:/core/save/all-external-js","markdowndemo.html","text/plain"
+		]
 	}
 }