Add 'getCommentAtCell' method into typings

[wertstein]

Context

On my recent commercial Angular project, I use getCommentAtCell method and it is missing in typings. I have to use a workaround like plugin['getCommentAtCell']().

How has this been tested?

I added getCommentAtCell method to typings file on my local machine and snippethot.getPlugin('comments').getCommentAtCell(1, 2); has no TSLint errors.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature or improvement (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Additional language file or change to the existing one (translations)

Checklist:

• My code follows the code style of this project,
• My change requires a change to the documentation.

[AMBudnik]

Thank you for sending the change and signing CLA.

[pnowak]

Thanks @wertstein for improvement.
But could you correct that this method can return not only a string but also an undefined/void one?

[pnowak]

Thank you for your correction.