fix: parenthesize ?? when it is combined with || or && - #1442

Fixes #1422.

?? cannot be combined with || or && without parentheses. The CoalesceExpression production only admits BitwiseORExpression operands, plus a nested CoalesceExpression on the left, so a || b ?? c and a ?? b || c are both SyntaxErrors.

needsParens decides this by comparing operator precedence, and ?? sits at the bottom of the table since #1095. That covers a ?? nested inside || or &&, which is why (a ?? b) || c prints correctly today. It does not cover the other direction: with ?? as the parent, pp > np is false and nothing is emitted.

So printing a ?? whose operand is a || or && produces output that does not parse:

LogicalExpression("??", LogicalExpression("||", a, b), c)   ->  a || b ?? c;
LogicalExpression("??", a, LogicalExpression("||", b, c))   ->  a ?? b || c;
LogicalExpression("??", LogicalExpression("&&", a, b), c)   ->  a && b ?? c;
LogicalExpression("??", a, LogicalExpression("&&", b, c))   ->  a ?? b && c;

All four throw when parsed:

"a || b ?? c" -> SyntaxError: Unexpected token '??'
"a ?? b || c" -> SyntaxError: Unexpected token '||'
"a && b ?? c" -> SyntaxError: Unexpected token '??'
"a ?? b && c" -> SyntaxError: Unexpected token '&&'

With this change they print as (a || b) ?? c, a ?? (b || c), (a && b) ?? c and a ?? (b && c).

The check is on the operator pair rather than on precedence, because this restriction is a grammar production rather than an ordering. ?? alongside a non-logical operator is unaffected: a ?? b + c, a + b ?? c and a ?? b ?? c still print without parentheses.

Verified by feeding every printed form through new Function to confirm it parses, and by checking the opposite direction is not over-parenthesized. Round-tripping unmodified source containing each shape comes back byte-identical.

Two tests are added. The first covers the four broken shapes and fails on main. The second locks in that ?? is left alone when it is not combined with || or &&, and passes on main, so it guards against over-correction rather than restating the fix.

npm test is clean: 779 passing, lint and build included.