* [shell] Adds a generic, portable shell library.

- Command line interface (cli) registration, parsing, and dispatch.
- Retargetable streamer object for input / output.
- Stdio-based streamer implementation for posix platforms.
- Default commands: help, echo.

Other changes during review:
- Build stdio streamer on UNIX only. ASF attribution.
- Mark nRF5 as embedded. Reorder includes.
- Add shell app to CI and VSCODE.
- Add missing build script.
- Fix standalone linker to use c++. Added base64 command.
- Converted all sources to C++ and wrapped in namespace chip::Shell.
- Fix shell_command naming consistancy. Shorten type names.
- Extend shell to support hierarchical subcommands.
- Added Doxygen.

Plus PR comment resolution:
- Moved location to `src/lib/shell`.
- Use test driver pattern for TestShell
- Use .cpp extension rather than .cc

* [shell] Review comments -- improve tokenizer space handling and unit tests.