chore(deps): update dependency webpack-cli to v4 (#692)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [webpack-cli](https://togithub.com/webpack/webpack-cli) | devDependencies | major | [`^3.3.11` -> `^4.0.0`](https://renovatebot.com/diffs/npm/webpack-cli/3.3.12/4.0.0) |

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### Release Notes

<details>
<summary>webpack/webpack-cli</summary>

### [`v4.0.0`](https://togithub.com/webpack/webpack-cli/blob/master/CHANGELOG.md#&#8203;400-httpsgithubcomwebpackwebpack-clicomparewebpack-cli400-rc1webpack-cli400-2020-10-10)

[Compare Source](https://togithub.com/webpack/webpack-cli/compare/v3.3.12...webpack-cli@4.0.0)

##### Bug Fixes

-   add compilation lifecycle in watch instance ([#&#8203;1903](https://togithub.com/webpack/webpack-cli/issues/1903)) ([02b6d21](https://togithub.com/webpack/webpack-cli/commit/02b6d21eaa20166a7ed37816de716b8fc22b756a))
-   cleanup `package-utils` package ([#&#8203;1822](https://togithub.com/webpack/webpack-cli/issues/1822)) ([fd5b92b](https://togithub.com/webpack/webpack-cli/commit/fd5b92b3cd40361daec5bf4486e455a41f4c9738))
-   cli-executer supplies args further up ([#&#8203;1904](https://togithub.com/webpack/webpack-cli/issues/1904)) ([097564a](https://togithub.com/webpack/webpack-cli/commit/097564a851b36b63e0a6bf88144997ef65aa057a))
-   exit code for validation errors ([59f6303](https://togithub.com/webpack/webpack-cli/commit/59f63037fcbdbb8934b578b9adf5725bc4ae1235))
-   exit process in case of schema errors ([71e89b4](https://togithub.com/webpack/webpack-cli/commit/71e89b4092d953ea587cc4f606451ab78cbcdb93))

##### Features

-   assign config paths in build dependencies in cache config ([#&#8203;1900](https://togithub.com/webpack/webpack-cli/issues/1900)) ([7e90f11](https://togithub.com/webpack/webpack-cli/commit/7e90f110b119f36ef9def4f66cf4e17ccf1438cd))

</details>

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packages/google-cloud-dialogflow/package.json

@@ -66,7 +66,7 @@
     "ts-loader": "^8.0.0",
     "typescript": "^3.8.3",
     "webpack": "^4.42.1",
-    "webpack-cli": "^3.3.11",
+    "webpack-cli": "^4.0.0",
     "@microsoft/api-documenter": "^7.8.10",
     "@microsoft/api-extractor": "^7.8.10"
   }