fix: parenthesize non-directive string literals in a directive prologue

[spokodev]

Bug

A string-literal expression statement at the start of a program or function body that is not a directive — parenthesized in the source, or built by a transform that didn't set directive — is generated without parentheses. Re-parsing then promotes it to a directive, which silently changes semantics (e.g. enabling strict mode) and can produce code that no longer parses:

import { generate } from 'astring'
import { parse } from 'acorn'

generate(parse('("use strict"); with ({}) {}'))
// => '"use strict"; with ({}) {}'
//    `with` is now inside a (newly-introduced) strict-mode prologue → SyntaxError on reparse

The same happens inside function bodies, and for things like ("use strict"); var x = 010; (octal becomes a strict-mode error). generate(parse(x)) is no longer idempotent for these inputs.

Cause

ExpressionStatement only parenthesizes for object/function ambiguity and assignment-to-pattern. It doesn't guard a string literal that sits at a directive-prologue position, so a non-directive string is emitted bare and is reparsed as a directive.

Fix

Track the directive prologue while writing a Program / BlockStatement body (it stays open across leading string-literal expression statements), and in ExpressionStatement parenthesize a string literal there only when it is not a directive — ESTree marks directives via node.directive, which acorn sets.

• Real directives (node.directive set) are emitted bare, exactly as before.
• Mid-body string statements are unaffected (the prologue is already closed), so existing output like debugger;\n"…"; is unchanged.
• Parenthesizing a string literal is always semantically safe, so the change can only ever add parentheses in the prologue case it targets.

Tests

Added src/tests/fixtures/syntax/directive.js (covered by the existing Syntax check exact round-trip test). It exercises a real directive (stays bare), a parenthesized prologue non-directive (kept parenthesized), a mid-body string (stays bare), and the same inside function bodies. On main this fixture fails (the parens are dropped); with this change it round-trips.

Note: I couldn't run npm test locally — [email protected] throws an uncaught exception at load under current Node (unrelated to this change; it fails the same way on a clean checkout). I verified the new fixture round-trips on the build, confirmed it fails on the published build (red→green), and diffed published-vs-patched output across every fixture plus ~43k random valid expressions: the only change is the intended prologue parenthesization, zero other differences.