env: placeholder completion

[rsteube]

Request

Environment variables can be dynamic.

Proposed solution

Support placeholders in variable name defiinitions and completions to support this.
Might be good to use variables or env as key instead of names.

variables:
  STATIC: static name
  DYNAMIC_<dyn>:  dynamic name
completion:
  variable:
    STATIC: ["one", "two"] 
    DYNAMIC_<dyn>: ["three", "four"]
  placeholder:
    dyn: ["five", "six"]

Anything else?

No response