Empty packages for native_assets_cli and c_compiler

[dcharkes]

Based on discussions we introduce two packages.

1. native_assets_cli which contains the command-line interface for building native assets.
   • Named native_assets_cli as this is is part of the Native asset feature.
     • ☂️ [vm/ffi] Native assets feature sdk#50565
   • Could possibly be named something like native_build_cli if we'd like to connect this with dart build name-wise.
     • Adding dart build sdk#51165
2. c_compiler which can compile C files with clang/msvc/xcodebuild/etc.
   • Named this way, so that a potential rust_compiler package can have consistent package naming.

Package names are not set in stone until we publish a first version of these packages.

This PR ensures we have the package layout, CI etc. set up correctly.

This PR mirrors the configuration from

• Empty package for package:cli_config tools#40

[mkustermann]

LGTM with comment

[dcharkes]

🙏