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.

tests/jq.test

@@ -553,6 +553,11 @@ def f(a;b;c;d;e;f): [a+1,b,c,d,e,f]; f(.[0];.[1];.[0];.[0];.[0];.[0])
 [1,2]
 [2,2,1,1,1,1]
 
+# Test precedence of 'def' vs '|'
+def a: 0; . | a
+null
+0
+
 # Many arguments
 def f(a;b;c;d;e;f;g;h;i;j): [j,i,h,g,f,e,d,c,b,a]; f(.[0];.[1];.[2];.[3];.[4];.[5];.[6];.[7];.[8];.[9])
 [0,1,2,3,4,5,6,7,8,9]