Unable to use "??=" in SSR

[Liamolucko]

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Describe the bug

Vite throws a syntax error (from acorn) when loading a module using ??= for SSR.

Reproduction

// index.js
let foo = null;
foo ??= 5;

// server.js
const vite = require("vite");
const server = vite.createServer().then((server) =>
  server.ssrLoadModule("/index.js")
);

System Info

• vite version: 2.1.0
• Operating System: Manjaro Linux
• Node version: 15.10.0
• Package manager (npm/yarn/pnpm) and version: pnpm 3.18.7

Logs (Optional if provided reproduction)

/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:35234
  var err = new SyntaxError(message);
            ^

SyntaxError: Unexpected token (2:6)
    at Object.pp$4.raise (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:35234:13)
    at Object.pp.unexpected (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:32926:8)
    at Object.pp$3.parseExprAtom (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34633:10)
    at Object.pp$3.parseExprSubscripts (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34436:19)
    at Object.pp$3.parseMaybeUnary (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34413:17)
    at Object.parseMaybeUnary (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:42955:29)
    at Object.pp$3.parseExprOp (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34373:41)
    at Object.pp$3.parseExprOps (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34350:91)
    at Object.pp$3.parseMaybeConditional (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34331:19)
    at Object.pp$3.parseMaybeAssign (/home/liam/src/playground/vite-project/node_modules/.pnpm/vite@2.1.0/node_modules/vite/dist/node/chunks/dep-6f1d3d8c.js:34299:19) {
  pos: 29,
  loc: Position { line: 2, column: 6 },
  raisedAt: 30
}

I think this should be fixed by updating acorn's ecmaVersion from 2020 to 2021 (or "latest").