fix: move install-theme.js to lib directory

[adamstankiewicz]

Description

The install-theme.js file does not get included when publishing to NPM as it's not explicitly mentioned in the package.json file's bin property.

Following the convention in frontend-build, the exposed paragon-scripts.js file now imports install-theme.js from a lib directory rather so the install-theme.js file is included when published to NPM.

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[edx-semantic-release]

🎉 This PR is included in version 21.1.1 🎉

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