Resolve shift/reduce conflict of 'def' vs '|'

This was an important conflict. In the following expression:

    def a: 0; . | a

Bison needs to decide between these two equally valid
parses:

    (def a: 0; .) | a
    def a: 0; (. | a)

For jq we want the second one, because the first results in
"a/0 is not defined". In the current parser the first parse
is a reduce and the second parse is a shift. Since Bison
prefers to shift in shift/reduce conflicts, we accidentally
got the correct behavior.

This commit adds a precedence level FUNCDEF which is lower
precedence than '|', meaning we explicitly choose the
correct parse.

Of course many unit tests already cover this case, but I
added one specifically for it.

parser.h

@@ -105,7 +105,8 @@ struct lexer_param;
     QQSTRING_TEXT = 296,
     QQSTRING_INTERP_START = 297,
     QQSTRING_INTERP_END = 298,
-    QQSTRING_END = 299
+    QQSTRING_END = 299,
+    FUNCDEF = 300
   };
 #endif
 /* Tokens.  */
@@ -151,6 +152,7 @@ struct lexer_param;
 #define QQSTRING_INTERP_START 297
 #define QQSTRING_INTERP_END 298
 #define QQSTRING_END 299
+#define FUNCDEF 300
 
 /* Value type.  */
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
@@ -162,7 +164,7 @@ union YYSTYPE
   jv literal;
   block blk;
 
-#line 166 "parser.h" /* yacc.c:1909  */
+#line 168 "parser.h" /* yacc.c:1909  */
 };
 # define YYSTYPE_IS_TRIVIAL 1
 # define YYSTYPE_IS_DECLARED 1