Support variable such as CURRENT_YEAR used by friendly-snippets

[jmetz]

There are a bunch of common system variables that spring to mind, including date-time related, username, email, or perhaps even the result of a shell command.

[estin]

Hi! It can be implemented with some restrictions.
I don't use snippets heavily and don't know common standards for variables in snippets.
This is "standard": https://code.visualstudio.com/docs/editor/userdefinedsnippets#_variables ?

[jmetz]

Yeah that looks pretty comparable to other snippet systems I've used before 👍