Parser: carry the declaration kind across nested parsing

Parser: carry the declaration kind across nested parsing

Previously, the kind of a declaration being parsed lived in a mutable
parser->var_type, written once when "var", "let" or "const" was seen and read
for every declarator in the list.  A declaration nested in a declarator
initializer overwrote it, so the declarators after it were recorded with the
wrong kind and lexical ones lost their per-iteration binding:

    let out = [];
    for (let a = function() { var z; }, b = 0; b < 2; b++) out.push(() => b);
    out[0]() + ',' + out[1]()

gave "2,2" instead of "0,1", because "b" was recorded as a var.

The fix is to carry the kind in njs_parser_stack_entry_t, as the [In]
parameter already is, so a continuation restores the kind in effect when it
was pushed.  njs_parser_stack_entry_t does not grow.