Fix preproc alternatives (elif, elifdef, else)

This commit merges functions "elseBlock" and "elifBlock" in a single
function "alternativeBlock". Any call to either function is replaced by
a call to "alternativeBlock", which cover all alternatives (elif,
elifdef and else). The goal is twofold:

1) Correct the grammar to allow all kind of alternatives independently
   of the "if" directive. Before the fix, "#if" alternative was
   generated by elseBlock only, and thus disallow "#elifdef" as a
   follow-up.
2) As a side effect, simplify the parser.

Author: Valentin Touzeau

grammar.js

@@ -1375,39 +1375,29 @@ function preprocIf(suffix, content, precedence = 0) {
     * @return {ChoiceRule}
     *
     */
-  function elseBlock($) {
+  function alternativeBlock($) {
     return choice(
       suffix ? alias($['preproc_else' + suffix], $.preproc_else) : $.preproc_else,
       suffix ? alias($['preproc_elif' + suffix], $.preproc_elif) : $.preproc_elif,
+      suffix ? alias($['preproc_elifdef' + suffix], $.preproc_elifdef) : $.preproc_elifdef,
     );
   }
 
-  /**
-    *
-    * @param {GrammarSymbols<string>} $
-    *
-    * @return {AliasRule | SymbolRule<string>}
-    *
-    */
-  function elifBlock($) {
-    return suffix ? alias($['preproc_elifdef' + suffix], $.preproc_elifdef) : $.preproc_elifdef;
-  }
-
   return {
     ['preproc_if' + suffix]: $ => prec(precedence, seq(
       preprocessor('if'),
       field('condition', $._preproc_expression),
       '\n',
       repeat(content($)),
-      field('alternative', optional(elseBlock($))),
+      field('alternative', optional(alternativeBlock($))),
       preprocessor('endif'),
     )),
 
     ['preproc_ifdef' + suffix]: $ => prec(precedence, seq(
       choice(preprocessor('ifdef'), preprocessor('ifndef')),
       field('name', $.identifier),
       repeat(content($)),
-      field('alternative', optional(choice(elseBlock($), elifBlock($)))),
+      field('alternative', optional(alternativeBlock($))),
       preprocessor('endif'),
     )),
 
@@ -1421,14 +1411,14 @@ function preprocIf(suffix, content, precedence = 0) {
       field('condition', $._preproc_expression),
       '\n',
       repeat(content($)),
-      field('alternative', optional(elseBlock($))),
+      field('alternative', optional(alternativeBlock($))),
     )),
 
     ['preproc_elifdef' + suffix]: $ => prec(precedence, seq(
       choice(preprocessor('elifdef'), preprocessor('elifndef')),
       field('name', $.identifier),
       repeat(content($)),
-      field('alternative', optional(elseBlock($))),
+      field('alternative', optional(alternativeBlock($))),
     )),
   };
 }