Add semantic accessors to macros

This way we can query the node and implement children-specific logic to
`content` and `alternative`. The nodes below `content` and `alternative`
may be Fortran nodes and not preprocessor directives!! So, for instance,
we want to use `&` as line separator instead of `\`.

Author: inaki-amatria

grammar.js

@@ -2353,7 +2353,7 @@ function preprocIf(suffix, content, precedence = 0) {
       field('condition', $._preproc_expression),
       optional($.preproc_comment),
       '\n',
-      content($),
+      field('content', content($)),
       field('alternative', optional(alternativeBlock($))),
       preprocessor('endif'),
       optional($.preproc_comment),
@@ -2363,7 +2363,7 @@ function preprocIf(suffix, content, precedence = 0) {
       choice(preprocessor('ifdef'), preprocessor('ifndef')),
       field('name', $.identifier),
       optional($.preproc_comment),
-      content($),
+      field('content', content($)),
       field('alternative', optional(alternativeBlock($))),
       preprocessor('endif'),
       optional($.preproc_comment),
@@ -2372,23 +2372,23 @@ function preprocIf(suffix, content, precedence = 0) {
     ['preproc_else' + suffix]: $ => prec(precedence, seq(
       preprocessor('else'),
       optional($.preproc_comment),
-      content($),
+      field('content', content($)),
     )),
 
     ['preproc_elif' + suffix]: $ => prec(precedence, seq(
       preprocessor('elif'),
       optional($.preproc_comment),
       field('condition', $._preproc_expression),
       '\n',
-      content($),
+      field('content', content($)),
       field('alternative', optional(alternativeBlock($))),
     )),
 
     ['preproc_elifdef' + suffix]: $ => prec(precedence, seq(
       choice(preprocessor('elifdef'), preprocessor('elifndef')),
       field('name', $.identifier),
       optional($.preproc_comment),
-      content($),
+      field('content', content($)),
       field('alternative', optional(alternativeBlock($))),
     )),
   };