chore(data-grid): change isCell function into a type guard and remove unnecessary type assertions

[abereghici]

change data grid isCell function into a type guard and remove unnecessary type assertions.

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🦋 Changeset detected

Latest commit: 25c84ea

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

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@twilio-paste/data-grid	Patch
@twilio-paste/core	Patch

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[TheSisb]

Thanks for the improvement! Really nice work!

Small fix for our changeset and I can merge this today

[abereghici]

Thanks for the improvement! Really nice work!

Small fix for our changeset and I can merge this today

done!