[core] feat(Utils): elementIsTextInput utility function

[adidahiya]

Changes proposed in this pull request:

Add a new utility function to the public API. We found some use cases for it in our internal codebases, so I'd like to expose it:

import { Utils } from "@blueprintjs/core";

Utils.elementIsTextInput(...);

[adidahiya]

[core] feat(Utils): elementIsTextInput utility function

Build artifact links for this commit: documentation | landing | table | demo

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