fix(types): switch to declaration file

[thomas-young1]

Description

Changes a file that causes a Typescript error when verbatimModuleSyntax is enabled to be a declaration files, like the rest of the files under types

• [Bug]: Enabling verbatimModuleSyntax breaks typechecking #487

Pull Request Type

Please select the option(s) that are relevant to this PR.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement (fixing a typo, updating readme, renaming a variable name, etc)