Better structured content support

[rampaa]

WPF TextBox does not support HTML and there is no plan to migrate to a solution like WebView2 to support it.

Given these constraints, in order to display structured content, JL needs to convert it to plain-text form. The current logic for that can be found at EpwingYomichanUtils.cs and the related JSON schema can be found at dictionary-term-bank-v3-schema.json.

I think the current logic can be enhanced further to make the structured content support better but I am not sure exactly how, thus the help wanted label. Detailed ideas and PRs are quite welcome.