Add method to script editor websocket that lists scripts available for subscription

[WolfGangS]

Description

Adds a method for the web socket server to allow 3rd party tools to query scripts available for subscription. Useful for the sl-vscode-plugin to be able to resubscribe after a connection loss (like the user updated vscode, or similar)

Also useful for plans to add support for a cli version of the plugin, to allow broader editor compatibility.

Related Issues

Issue Link: relates to secondlife/sl-vscode-plugin#43

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Checklist

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Additional Notes

Discussed with @Rider-Linden before, so likely more his wheelhouse for review.

I suspect there may be some issue I am missing with my changes to LLScriptEditorWSServer::unsubscribeConnection but they (or something like them) are needed else a websocket connection closing flushes scripts from the subscription list, even though they are still open in viewer, and on disk in the temp directory.