[MP-2044] Add support for theme data

[SamborGornicz]

Added support for retrieving theme values through the getTheme method and the change_theme event:

• Each widget now includes a getTheme method, which will return the current parent theme value based on the initial theme specified in the widget URL via the ?theme=dark|light query parameter and on data provided from change_theme.
• Added TypeScript definitions for the change_theme event, which is triggered whenever the theme changes in the Agent App.

[changeset-bot]

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Latest commit: 00333a2

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[arturfracala]

Overall, looks good 👍🏿

[arturfracala]

Are these libraries not being used?

[SamborGornicz]

No, they are not used in the agent-app-sdk, but they are present in the widget-sdk-core. I assume they might have been used in the past, or perhaps the package.json file was simply copied from widget-sdk-core.