Add commands

[gavr123456789]

Add commands as in Philip's VLS.
I'm not sure what they're called, but I mean that:

[albfan]

@gavr123456789 I guess you mean https://github.com/philippejer/vala-language-client-alpha right?

What commands you want to see implemented?

[Prince781]

These are the commands I have in mind for now:

• vala.showHiddenSymbol - show the symbol that is hidden by the symbol at the cursor
• vala.showBaseSymbol - show the symbol that is implemented by (or overridden by) the symbol at the cursor
• vala.restartServer - restarts the language server

Any others?