Move GN root into //core/libdeno

[ry]

As of #2865, GN is only used now to build V8 and libdeno - the rest of the build is handled by cargo. Therefore it would be nice to not pollute the root directory with various GN/Ninja related directories, symlinks, and files that most will not need to look at.

Basically the following "git mv" actions need to be made:

//build -> //core/libdeno/build
//.gn -> //core/libdeno/.gn
//build_overrides -> //core/libdeno/build_overrides
//buildtools -> //core/libdeno/buildtools

but this will also require adjusting our build scripts and other tooling.

[chrmoritz]

I'm currently looking into this.

Current progress in: https://github.com/chrmoritz/deno/commits/libdeno_gn_root

=> #2938