Remove get-port and add custom getPort function with tests

[MylesBorins]

Removes the 'get-port' package and replaces its functionality with a custom getPort function in the test/util.mjs file. Adds a new test file to ensure the getPort function works as expected.

• Removes "get-port" from devDependencies in package.json, eliminating the package from the project.
• Implements a new getPort function in test/util.mjs using the Node.js net module to find an available port, replacing the previous usage of the get-port package.
• Adds a new test file test/test-getPort.mjs to verify that the getPort function correctly returns an available port and handles error scenarios, aiming for 100% test coverage.
• Updates rollup.config.mjs by removing the retainImportExpressionPlugin function and its usage, which was specifically designed to handle dynamic imports of the get-port package, reflecting the removal of get-port from the project.

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