Bump @blockly/theme-modern from 2.1.10 to 2.1.13

Bumps [@blockly/theme-modern](https://github.com/google/blockly-samples/tree/HEAD/plugins/theme-modern) from 2.1.10 to 2.1.13.
- [Release notes](https://github.com/google/blockly-samples/releases)
- [Commits](https://github.com/google/blockly-samples/commits/@blockly/theme-modern@2.1.13/plugins/theme-modern)

Signed-off-by: dependabot[bot] <support@github.com>