Add label metadata to symbolic variables

[ven-k]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the ScioML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

• label is one word name of parameters/variables.
• label can be added to Symbolic variables inside @mtkmodel too.

@variables l [label = "Label"]

@mtkmodel Resistor begin
    @parameters begin
          R [label = "Resistance"]
    end
    ...
end