chore(deps): update dependency ts-loader to v8 (#357)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [ts-loader](https://togithub.com/TypeStrong/ts-loader) | devDependencies | major | [`^7.0.0` -> `^8.0.0`](https://renovatebot.com/diffs/npm/ts-loader/7.0.5/8.0.0) |

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### Release Notes

<details>
<summary>TypeStrong/ts-loader</summary>

### [`v8.0.0`](https://togithub.com/TypeStrong/ts-loader/blob/master/CHANGELOG.md#v800)

[Compare Source](https://togithub.com/TypeStrong/ts-loader/compare/v7.0.5...v8.0.0)

-   [Support for symlinks in project references](https://togithub.com/TypeStrong/ts-loader/pull/1136) - thanks [@&#8203;sheetalkamat](https://togithub.com/sheetalkamat)!
-   `ts-loader` now supports TypeScript 3.6 and greater **BREAKING CHANGE**

</details>

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packages/google-cloud-asset/package.json

@@ -61,7 +61,7 @@
     "null-loader": "^4.0.0",
     "pack-n-play": "^1.0.0-2",
     "sinon": "^9.0.1",
-    "ts-loader": "^7.0.0",
+    "ts-loader": "^8.0.0",
     "typescript": "^3.8.3",
     "webpack": "^4.42.0",
     "webpack-cli": "^3.3.11"