Added type of string array to brush targets.

[vargemp]

New Pull Request

When using Typescript (react-apexcharts in my case) there is no posibillity to target multiple charts with brush. This PR should fix it.

*that's my first PR on Github ever, also first contribution to someone's package, so if I've done something wrong please correct me.

Fixes #3917 (issue)

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• This change requires a documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• My changes generate no new warnings
• New and existing unit tests pass locally with my changes