Add typing to Command in Hello World Template

[Jonas Helming]

Description

We should add proper typing when registering the HelloWorldCommand for downstream adopters more easily defining commands:

generator-theia-extension/templates/hello-world/contribution.ts

Line 5 in 2b0ad9e

 export const <%= params.extensionPrefix %>Command = { 

Should be updated to:

export const <%= params.extensionPrefix %>Command: Command = {