chore: bump acorn to 8.0.1

[JLHwung]

closes #11186
closes #10227

This PR aims to bump acorn so ?., ??, 4_2, x ??= y should not throw syntax error: It is part of ES2021.

This PR does not backport evaluations support in #11221. I think it is good enough for webpack@4. To further optimize expression related to these new syntax features, please upgrade to webpack@5. But the bottom line here is that webpack@4 should not throw for ES2021 syntax, especially when webpack 5 is not released yet.

What kind of change does this PR introduce?

bugfix

Did you add tests for your changes?

No. The acorn tests itself should be sufficient.

Does this PR introduce a breaking change?

Yes. See https://github.com/acornjs/acorn/blob/master/acorn/CHANGELOG.md#700-2019-08-13 Though acorn v7 to v8 is not breaking webpack 4.

What needs to be documented once your changes are merged?

Nothing.

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[alexander-akait]

@JLHwung I think we don't need this because webpack@5 ETA is 2020-10-10, so better to update acorn for webpack@5 to 8 version

[JLHwung]

@evilebottnawi I think it will be great to bump acorn on webpack@4 because after webpack@5 is released, it will take a while for communities to catch up. During this period, people may use latest syntax and the error thrown from webpack@4 is thus confusing:

Module parse failed: Unexpected token (2:10)
File was processed with these loaders:
 * ./node_modules/ts-loader/index.js
You may need an additional loader to handle the result of these loaders.
| var a = { b: 2 };
> var c = a?.b;

This is not some customized JS syntax that must be handled by loader. In this way I consider bumping acorn as a bugfix to webpack@4.

[sokra]

It's a breaking change and we can't do it for webpack 4.

[JLHwung]

@sokra I see. If it is a breaking change because

Changes the node format for dynamic imports to use the ImportExpression node type, as defined in ESTree.

It will be great to make it clear at the webpack@4 docs, that webpack@4 supports only ES2019 features. Any ES2020 features is not supported and should be handled by additional loader.

Should we discourage practise like

"resolutions": {
    "acorn": "8.0.1"
},

?

[webpack-bot]

@JLHwung The most important CI builds failed. This way your PR can't be merged.

Please take a look at the CI results from travis (error) and appveyor (failure) and fix these issues.