fix(eslint-plugin): [prefer-string-starts-ends-with] Check negative indices in the second position for slice

[ginger-kang]

fixes #2688

[typescript-eslint]

Thanks for the PR, @ginger-kang!

typescript-eslint is a 100% community driven project, and we are incredibly grateful that you are contributing to that community.

The core maintainers work on this in their personal time, so please understand that it may not be possible for them to review your work immediately.

Thanks again!

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[bradzacher]

LGTM - thanks for fixing this!