Add a support for package custom theme for Express Build mode

[mshabarov]

Custom themes packaged as a JAR dependency (parent theme and re-used theme) should work in Express Build mode.
Acceptance criteria:

• Packaged theme feature should work in Express Build mode as described in the docs here https://vaadin.com/docs/latest/styling/custom-theme/custom-theme-packaging and https://vaadin.com/docs/latest/styling/custom-theme/custom-theme-configuration/#extending .
• Adding a packaged theme shouldn't require a new dev bundle generation until the packaged theme defines any external assets to be added to the project (assets block in theme.json).

Possible implementation details:

• Flow needs to look at the frontend/generated/jar-resources/themes at build time and add the link tag to reference the styles.css on the page for each packaged theme.
• Static file server should serve the CSS and assets defined in the packaged theme from frontend/generated/jar-resources/themes/my-packaged-theme.
• Flow should spot an external assets included into packaged theme and trigger a new bundle generated to serve these assets from applicaiton bundle during the next runs.

[vaadin-bot]

This ticket/PR has been released with Vaadin 24.0.0.alpha10 and is also targeting the upcoming stable 24.0.0 version.