Enable core module embedding by default in CMake.

[kanashimia]

It seems that release workflows were unexpectedly broken when STDLIB was renamed to CORE_MODULE.
Changed the option default because I think it just makes more sense. Removed explicit calls to enable it. Fixed docs.

Should SLANG_EMBED_CORE_MODULE_SOURCE be disabled too? Does it do anything useful?

As reported in #5631

NOTE: I haven't really tested it at this point, but should work.

[CLAassistant]

All committers have signed the CLA.

[csyonghe]

Looks good, thanks for your contribution!

[csyonghe]

The only reason to enable SLANG_EMBED_CORE_MODULE_SOURCE is so that the language server can provide useful documentation in hovering hints. We could disable this for most builds, and enable it only when building for language server, but that likely needs to come as its own PR. We can change the language server code to fetch the core module source from a separately deployed file that ships with slangd in the vs extensions, and get rid of this embeding in slang.dll to reduce binary size.

[jkwak-work]

I appreciate fixing the mistake I made.
I guess I didn't carefully check directories whose name starts with dot, "."