Added example using Buck with Visual Studio Code on Windows

Summary:
1. I made the default platform in the prelude public so it can be inherited from. I don't know why it wasn't already like that. Maybe I'm misunderstanding how to use it
2. Not sure if I'm using the config_setting and constraint_setting properly since it seems duplicated.
3. The script to generate the compile_commands.json was more or less copied from facebook/buck2#510.

There are still a lot of improvements to be made but I think this is good enough to be merged:
1. With MSVC the compilation should probably be done with /Z7 by default and then, if the "stripped" sub-target is asked for, that flag is removed.
2. The cxx toolchain needs the compilation options looked at, not sure if Linux's defaults work properly.
3. The Install.ps1 and Copy.ps1 need to be ported to Linux.

X-link: facebook/buck2#517

Reviewed By: KapJI

Differential Revision: D53692733

Pulled By: JakobDegen

fbshipit-source-id: 8d5962084831a3cccde128f0aa0360a1aaf72004

platforms/BUCK.v2

@@ -9,6 +9,7 @@ execution_platform(
     cpu_configuration = host_configuration.cpu,
     os_configuration = host_configuration.os,
     use_windows_path_separators = host_info().os.is_windows,
+    visibility = ["PUBLIC"],
 )
 
 prelude.constraint_setting(

toolchains/cxx.bzl

@@ -67,7 +67,6 @@ def _system_cxx_toolchain_impl(ctx: AnalysisContext):
         shared_library_name_default_prefix = ""
         shared_library_name_format = "{}.dll"
         shared_library_versioned_name_format = "{}.dll"
-        additional_linker_flags = ["msvcrt.lib"]
         pic_behavior = PicBehavior("not_supported")
     elif ctx.attrs.linker == "g++" or ctx.attrs.cxx_compiler == "g++":
         pass