citeproc-js: Add space after left margin in plain-text mode

This adds a space after the the number in some numeric styles ("[1]" in
IEEE style, "1." in Nature) when pasting bibliography entries into a
plain-text editor.

Some styles (ACS, AMA, Vancouver) will need to be updated to remove
their hard-coded spaces to avoid double spaces after this change.

Patch from @zepinglee

Fixes #2633

chrome/content/zotero/xpcom/citeproc.js

@@ -22261,7 +22261,7 @@ CSL.Output.Formats.prototype.text = {
         return "\n"+str;
     },
     "@display/left-margin": function (state, str) {
-        return str;
+        return str + " ";
     },
     "@display/right-inline": function (state, str) {
         return str;