Add electric dedent to lua mode

[ghost]

The lua mode currently adds a level of indentation when opening up a new
scope, but closing the scope doesn't remove a level. This change gives
opening scope indents a respective dedent.

[marijnh]

This breaks indentation on lines after else or elseif. I don't think there's any reason to touch dedentTokens, just adding the appropriate regexp as electricInput should be enough.

[ghost]

This is the behaviour I'm going for: https://webmshare.com/play/q5gmb

So I think the dedent keywords need separating out to differentiate between the indentation of the next scope and that of the line being typed.

I hacked together the .webm by using 'then' for the if/elseif indentation and then treating 'else' as a special case. Maybe there's a nicer way?

--- lua_old.js	2018-05-25 08:16:59.411017519 +0100
+++ lua_new.js	2018-05-25 08:24:47.534394133 +0100
@@ -63,9 +63,10 @@
                          "true","function", "end", "if", "then", "else", "do",
                          "while", "repeat", "until", "for", "in", "local" ]);
 
-  var indentTokens = wordRE(["function", "if","repeat","do", "\\(", "{"]);
-  var dedentTokens = wordRE(["end", "until", "\\)", "}"]);
-  var dedentPartial = prefixRE(["end", "until", "\\)", "}", "else", "elseif"]);
+  var indentTokens = wordRE(["function", "then","repeat","do", "\\(", "{"]);
+  var dedentKeywords = ["end", "until", "\\)", "}", "else", "elseif"];
+  var dedentTokens = wordRE(dedentKeywords);
+  var dedentPartial = prefixRE(dedentKeywords);
 
   function readBracket(stream) {
     var level = 0;
@@ -138,7 +139,7 @@
       }
       if ((style != "comment") && (style != "string")){
         if (indentTokens.test(word)) ++state.indentDepth;
-        else if (dedentTokens.test(word)) --state.indentDepth;
+        else if (dedentTokens.test(word) && word !== "else") --state.indentDepth;
       }
       return style;
     },
@@ -148,6 +149,7 @@
       return state.basecol + indentUnit * (state.indentDepth - (closing ? 1 : 0));
     },
 
+    electricInput: new RegExp("\\b" + dedentKeywords.join("|") + "\\b", "g"),
     lineComment: "--",
     blockCommentStart: "--[[",
     blockCommentEnd: "]]"