Feat: command proxy for addons api

[lifeart]

usecase:

currently we have proxy command, in extension itself, it allow execute any VSCode command, and return execution results into any command

https://github.com/lifeart/vscode-ember/blob/master/src/extension.ts#L98

we need to create api to return result back to extension server and pass it down to addon

for example:

1.) addon: execute command ["create-file", "els:after-create-file"]
2.) server: execute command ["proxy-command", "create-file", "els:after-create-file" ]
3.) vscode: execute command "create-file"
4.) passing result back to extension
5.) extension passing result back to server
6.) server passing result back to "els:after-create-file", registered in addon

addon_command -> api_command -> extension_proxy -> (vscode command evaluation) -> extension_proxy -> api_command -> other_addon_command