Cannot install with yarn

```
yarn add highcharts-rounded-corners
yarn add v1.3.2
[1/4] 🔍 Resolving packages...
error An unexpected error occurred: "https://registry.yarnpkg.com/Highcharts: Not found".
info If you think this is a bug, please open a bug report with the information provided in "/Users/xxx/yyy/yarn-error.log".
info Visit https://yarnpkg.com/en/docs/cli/add for documentation about this command.
```

I think this is due to the incorrect capitalisation of the `highcharts` dependency (it's `Highcharts` in the `package.json`).

I'll submit a PR to fix this.

Start by inspecting the dependency declaration in package.json and reproduce the reported yarn add highcharts-rounded-corners command. Confirm the package name resolves with the expected casing, then verify that installation completes successfully without the registry error.

Written by the indexing model from the issue text.