API to represent dead / inactive code

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> Language plugins have to resort to custom LSP extensions to achieve a dimming effect for inactive code like in other IDEs: >  I'd suggest adding an `inactive` or `disabled` [SemanticTokenModifier](https://microsoft.github.io/language-server-protocol/specifications/specification-current/#textDocument_semanticTokens) and using it to style the code like in the picture. The only question is how to make sure the whole line appears differently (_iff_ the whole line is affected) and not just the portion of each inactive line covered by text, which may look weird.