text: Title: Support creating variable name based on another variable like `${${VAR}}` in Variables section
  Body: Example:
  ```robotframework
  *** Variables ***
  ${X}       Y    # Normal assignment. Creates ${X} with value Y.
  ${${X}}    Z    # Assignment based on another variable. Creates ${Y} with value Z.
  ```
  
  This is likely not that widely needed feature, but we plan to support this approach with the new `VAR` syntax (#3761) and we also have an issue about supporting it when creating variables based on keyword return values (#4545). Especially the `VAR` usage will be very close to what proposed above and supporting the same in the Variables section is good for consistency.
  
  ```
  VAR    ${X}       Y    # Normal assignment. Creates ${X} with value Y.
  VAR    ${${X}}    Z    # Assignment based on another variable. Creates ${Y} with value Z.
  ```
  
  Implementation shares most of the same code that ´VAR` uses, so the extra effort is really small.