fix: keep parentheses around an optional chain used as a member/call/new target

[sarathfrancis90]

generate drops the parentheses that terminate an optional chain when the chain is the object/callee of a member access, call, or new:

import { generate } from "astring";
import { parse } from "acorn";
const opts = { ecmaVersion: 2023 };
generate(parse("(a?.b).c", opts));      // "a?.b.c;"      ← wrong
generate(parse("new (a?.b)()", opts));  // "new a?.b();"  ← not even valid syntax

(a?.b).c and a?.b.c aren't equivalent: the parenthesized form ends the chain at a?.b, so .c runs unconditionally and throws when a is nullish; the unparenthesized form extends the chain and short-circuits to undefined.

The cause is that ChainExpression shares precedence (19) with MemberExpression/CallExpression/NewExpression, so the precedence-only check never parenthesizes it. This wraps the chain when it's the object/callee of those three nodes. a?.b.c (no terminating parens) is unaffected. Added the cases to the object syntax fixture.