WIP: Merge 'select_case' and 'default_case'

Not sure whether I like this change or not... IMO this might be too lax.

grammar.js

@@ -2,10 +2,6 @@ function commaSeparated(elem) {
   return seq(elem, repeat(seq(",", elem)), optional(","))
 }
 
-function trailingCommaSeparated(elem) {
-  return repeat(seq(elem, ","))
-}
-
 module.exports = grammar({
   name: "bp",
 
@@ -153,24 +149,16 @@ module.exports = grammar({
 
     _select_cases: ($) => seq(
       "{",
-      optional(trailingCommaSeparated($.select_case)),
-      // default *must* be the last one, enforced at parse-time...
-      optional(seq(alias($.default_case, $.select_case), ",")),
+      optional(repeat(seq($.select_case, ","))),
       "}",
     ),
 
     select_case: ($) => seq(
-      field("pattern", $._string_literal),
+      field("pattern", choice(alias("default", $.default), $._string_literal)),
       ":",
       field("value", $._case_value)
     ),
 
-    default_case: ($) => seq(
-      field("pattern", alias("default", $.default)),
-      ":",
-      field("value", $._case_value),
-    ),
-
     _case_value: ($) => choice(
       alias("unset", $.unset),
       $._expr,