Support creating variable name based on another variable like ${${VAR}} in Variables section

[pekkaklarck]

Example:

*** 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.

[pekkaklarck]

Now that we add this support to Variables, VAR and to keyword return values, we could consider making the nested syntax valid everywhere where variables are created. That would allow usages like this:

FOR    ${${x}}    IN    a    b    c

EXCEPT    AS    ${${x}}

[Arguments]    ${${x}}

I don't see any use case for any of these and the latest one would be rather confusing and make resolving named argument usages complicated. Consistently supporting same syntax everywhere would have benefits, but I don't believe they are big enough here. Anyway, we can return to this in the future if needed.