Windows: Make MSVC debug CRT opt-in (debug_crt=yes)

[akien-mga]

Follow-up to #69294 which enabled it automatically for dev_build=yes, but this seems to cause some issues.
Those issues should likely be debugged but until then this can stay opt-in

[YuriSizov]

Can confirm that now by default builds don't cause extreme slowdowns. This is true even with the language server doing its thing on startup when you have Godot Tools enabled in VS Code.

However, with debug_crt=yes I also don't get slowdowns anymore... I've tried rebuilding it 2 times, without the flag then with it. It does a complete rebuild, and yet it doesn't cause the issue anymore. I've tried adding a print statement to make sure that the build goes through the correct branch, and it does, it prints as expected. And I do get that "abort" message when I close one of my projects.

Going back to the current master doesn't cause a rebuild at all (because all the compiler flags match, I guess?). So I went to the last commit before you've introduced your original change, built that, checked that there was no issue, switched to the latest commit in master, and built again, and it worked still.

So I'm confused... I've tested a few different commits earlier today, trying to pinpoint the regression. It was absolutely repeatable every time I tried the one from #69294 or later, but it was clear for multiple commits before that. What.

[YuriSizov]

I guess I'll tentatively approve this, since it restores the status quo and makes it unlikely for others to run into the same issue, until we understand the cause.

[Zylann]

As I suggested here #69294 (comment)
I think you dont get the slowdown anymore because it could have just been shaders compilation (which typically uses all threads). I get that too. After caching, startup is faster. No idea why the language server would be involved in this though, I havent seen it. Overall, it is normal to see a bit of slowdown, since we simply weren't using debug Godot with "release" C++ runtime before, and some thirdparty libs might even have been skipping debug checks.