Scaladoc: Support for Interactive Feature Toggles

[bracevac]

Enhance scaladoc to support dynamic toggles of features like capture checking (and explicit nulls) in the browser. The recently merged PR #23607 for capture-checking currently generates only static content that renders cc-specific types by default. A command-line option suppresses the cc-related content.

1. CC-related typesetting is presently shoehorned into the pre-existing classes in api.scala. This IR should be extended to support new classes of (toggleable) features. Otherwise, it's not easily possible to distinguish this information in the HTML rendering code.

2. Adapt the HTML rendering code accordingly.

Together with #23742, this might be a good semester or internship project.