[plug-in] 4084 show refs command

[tsmaeder]

Fixes #4084.

1. Separates the "Command" objects used in the Theia plugin API for registration of commands and invocation of commands into two separate interfaces: CommandRegistration and Command. The "Command" interface is compatible with the related vscode API interface. The unnecessary fields in the separated interfaces have been made deprecated. They will be removed after a period of deprecation. The new code should be compatible with the old version.
2. Introduces "KnownCommands" where well known command invocations can be mapped to other command ids and parameters can be converted.
3. Removed "CommandConverter", as I don't think it is needed.

[benoitf]

maybe a comment on https://github.com/theia-ide/theia/blob/master/CHANGELOG.md in breaking changes (or deprecation notice)