Add fallback Html panel support for markdown rendering

[ZacSweers]

Android Studio frustratingly ships with a broken version of the embedded markdown plugin, which prevents use of the modern JCEF API we were using. This adds a fallback implementation for when that's not available and just uses a simpler html panel. Code blocks don't work and it's a little plain, but it's otherwise ok.

[github-actions]

Review:

1. Remove unused imports: Remove the unused imports at the top of the file to improve code readability.

import com.intellij.openapi.diagnostic.logger
import com.intellij.ui.dsl.builder.panel
import com.intellij.ui.dsl.gridLayout.HorizontalAlign
import com.intellij.ui.dsl.gridLayout.VerticalAlign
import com.intellij.ui.jcef.JBCefApp
import com.intellij.util.ui.HtmlPanel
import com.intellij.util.ui.JBUI

2. Extract the creation of the panel into a separate method: Extract the creation of the panel into a separate method to improve code readability and maintainability. This method can be called createPanel and it should take the project, file, and html as parameters and return the created panel.

private fun createPanel(project: Project, file: VirtualFile, html: String): JComponent {
    // Create the panel here
}

3. Use the extracted method in the main method: Replace the existing code for creating the panel with a call to the extracted method.

val panel = createPanel(project, file, html)
return panel

4. Simplify the panel creation for the legacy mode: In the legacy mode, instead of using the panel DSL, you can directly create an HtmlPanel and set its body to the generated HTML.

val htmlPanel = HtmlPanel().apply {
    isVisible = true
    border = JBUI.Borders.empty(16)
    setBody(html)
}
return htmlPanel

5. Remove unnecessary logging: Remove the logging statements that are not necessary for the functionality of the code.

// Remove these lines
private val logger = logger<WhatsNewPanelContent>()
logger.debug("Using JCEFHtmlPanelProvider")
logger.debug("Using HtmlPanel")

By following these steps, the code can be improved for readability and maintainability.