Parenthesize let [-prefixed expression statements

[sarathfrancis90]

An ExpressionStatement cannot begin with the token sequence let [, because the parser then treats it as the start of a lexical declaration. escodegen already guards the analogous {, function and class cases, but not this one, so it produced broken output for expressions whose leading identifier is let followed by a computed member access.

For example, given the AST for:

(let[a]) = b;
(let[a])();

escodegen generated:

let[a] = b;   // reparses as a `let [a] = b` destructuring declaration
let[a]();     // SyntaxError on reparse

The first silently changes an assignment into a declaration; the second is not valid JavaScript at all.

The fix adds a small isLetPrefixed check alongside the existing statement-start guards and wraps the expression in parentheses when its generated text starts with let [. Identifiers that merely start with let (e.g. lettuce[a]) and real let declarations are unaffected.

Tested with npm test (all existing tests still pass) plus a new round-trip fixture under test/compare-acorn-es2021/ that parses the affected forms, regenerates them and confirms the output reparses to an identical AST.