[ast] better display of string matrices

[ast] better display of string matrices

https://bugzilla.scilab.org/show_bug.cgi?id=16192

example:

--> x=["Mercury","Gemini","Apollo";"Skylab ABCDEF","Skylab B","ISS"]
 x  =

  "Mercury"        "Gemini"    "Apollo"
  "Skylab ABCDEF"  "Skylab B"  "ISS"

restored natural wrapping for very long strings, which allows direct
selection in console:

--> x="";for i=1:400,x=x+string(modulo(i,10)); end

--> x
 x  =

  "123456789012345678901234567890123456789012345678901234567890123456789
012345678901234567890123456789012345678901234567890123456789012345678901
234567890123456789012345678901234567890123456789012345678901234567890123
456789012345678901234567890123456789012345678901234567890123456789012345
678901234567890123456789012345678901234567890123456789012345678901234567
8901234567890123456789012345678901234567890"

Change-Id: I0103c8e52e4476c446f28a79de217532adc06ab0