chore(export): spline classes for custom spline tool creation

[JSweet314]

Context

This PR exposes the spline classes underlying the SplineROITool so that library users may create their own spline tools.

Changes & Results

Add new namespace, splines, which exports spline classes.

Testing

Import a spline class! :)

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,
  etc.)

• 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.

Tested Environment

• [] "OS:
• [] "Node version:
• [] "Browser:

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