Support llmGenerated property on CodeAction

[StellaHuang95]

Addresses microsoft/vscode-languageserver-node#1555

• Added llmGeneratedSupport to CodeActionClientCapabilities.
• Added llmGenerated to CodeAction.
• Marked both of them as proposed.

[mfussenegger]

Wouldn't it be more flexible if this were just a source hint, which can be an arbitrary string, or an enum+string for some well defined code action sources where llm is one of them?
E.g. if LLM gets replaced by some newer tech, there wouldn't be need to introduce a xyGenerated property.

Then this also wouldn't need a client capability (because if the client shows a fancy icon or something shouldn't matter for servers)

[StellaHuang95]

Thanks for the feedback, I've updated to use CodeActionTag as suggested by Dirk. Please let me know if there's anything else you'd like me to address

[dbaeumer]

@StellaHuang95 we should use the same wording tweaks as proposed in microsoft/vscode-languageserver-node#1557