Bump @blockly/theme-modern from 2.1.17 to 2.1.18 (#4944)

Bumps [@blockly/theme-modern](https://github.com/google/blockly-samples/tree/HEAD/plugins/theme-modern) from 2.1.17 to 2.1.18.
- [Release notes](https://github.com/google/blockly-samples/releases)
- [Commits](https://github.com/google/blockly-samples/commits/@blockly/theme-modern@2.1.18/plugins/theme-modern)

---
updated-dependencies:
- dependency-name: "@blockly/theme-modern"
  dependency-type: direct:development
  update-type: version-update:semver-patch
...

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