fix(LengthTool): _dragCallback element reference

[alex-stout]

Fixes the LengthTool's _dragCallback() method to fix a missing element reference.

Also cleans up some typescript typing.

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Changes & Results

Testing

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
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• [] I have run the yarn build:update-api to update the API documentation, and have
  committed the changes to this PR. (Read more here https://www.cornerstonejs.org/docs/contribute/update-api)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

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[sedghi]

I don't understand the value of this PR