Fixes LSP connection error when launched in a separate thread

[azuloo]

Fixes #79525 . I went with set_current_thread_safe_for_nodes(true) and it fixes the problem but I'm not sure if it doesn't introduce any security issues. But from what I've found out - GDScriptLanguageServer has no thread processing (current_process_thread_group = nullptr) and here we're left with two choices:

• only access nodes that are not part of a scene tree (that's not happening in out case) and
• mark a current thread as 'safe' (which is what this PR does)

Please, let me know if I'm missing some other possible ways of fixing this.

[RandomShaper]

This is good enough, but maybe not as good as it should in the long term. Let me explain.

If the LSP was working well, silencing the errors (what this PR does) will not make it any worse, but taking it back to the level of functionality it used to enjoy. In fact, the idea with set_current_thread_safe_for_nodes() was to also give a shortcut in cases like this to at least keep them working as they used to do without adding the obligation of a deep analysis of threading correctness right away.

Now, is this an opportunity to realize actual threading issues that warrant additional work? I can't answer this myself due to my lack of familiarity with what the LSP does.

[akien-mga]

Thanks!

[YuriSizov]

Cherry-picked for 4.1.3.