fix(getColormap): add opacity mapping to findMatchingColormap return object

[Sofien-Sellami]

Context

The getProperties function currently returns the colormap name, but it does not include the opacity.

Changes & Results

• Added the opacity mapping to the findMatchingColormap function's return object.
• The function now returns an object that conforms to the ColormapPublic type, including the name and opacity properties.

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,
  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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