Generate fields for markers in REPL so compiler plugin can extract the schema

[koperagen]

without this change plugin cannot see schemas from previous cells, generated by REPL:

val df = DataFrame.read("...")
%%
val df1 = df.add("a") { 42 } 
// only df1.a is available here, because from plugin POV df has empty schema

After this PR generated markers (DataFrame<T>) will have unified structure, so schema can be extracted from both REPL and plugin generated ones