Execute client side command

We are providing some code actions, and some of them should be invoked on client side (like opening a new panel to display some guidelines about diagnostics).

The official workflow specified with protocol v3 is:

• client invokes textDocument/codeAction
• server returns an array of Command
• user picks one command
• client invokes workspace/executeCommand
  So we end up on LS side, with no way to invoke a command on client side.

VSCode has a specific behavior. If a returned Command matches a command declared on VSCode side, it will not call workspace/executeCommand but instead directly invoke the client side command.

I find this very convenient, but this is not what is defined in protocol V3, and consequently not portable (Atom doesn't work like this for example, see https://github.com/atom/atom-languageclient/issues/183).

Proposals:

• officially support this behavior, but this maybe a bit too "magic" (what if client doesn't have a concept of command? can the LS be sure the command exists on client side?)
• add a new executeCommand operation, in the direction LS -> client, allowing a server to explicitly request a client to execute a command (with proper capability declaration, error handling, ...)
• to avoid extra round trip, maybe add a flag on the Command returned by textDocument/codeAction to precise if this is a client side or a server side command

Start by reviewing the protocol v3 code-action and workspace/executeCommand definitions, then compare the VSCode behavior described here with the Atom language client issue. The issue presents several competing proposals rather than a settled change. Done would require an agreed, portable client-side command mechanism with capability declaration and error handling.