Support for Stata 18

[JanMarvin]

Stata 18 is out and it ships two new file formats

• https://www.stata.com/help.cgi?dta_120
• https://www.stata.com/help.cgi?dta_121 (for more than 32k variables)

The clue appears to be something Stata calls alias variables. Though the dta help is a bit scarce in regard of these variables. The file format should be identical to 118/119 aside from the alias variable, therefore it might be enough to simply treat 118 like 120 and 119 like 121:

                         Stata 
           typ          meaning      Description
-----------------------------------------------------------------
         65525          alias        reference to variable in another frame

It might be some kind of memory pointer or external reference similar to an obnoxious spreadsheet software, but either it is not (yet) documented or I'm simply missing it.

[Edit:] https://www.stata.com/new-in-stata/alias-variables-across-frames/
[Edit2:] https://www.stata.com/help.cgi?fralias