[slang] Make string simple type

[likeamahoney]

To support such sample from SystemVerilog LRM section 10.9.2:

module top;
   typedef struct {
      logic [7:0] a;
      bit b;
      bit signed [31:0] c;
      string s;
   } sa;
   
   sa s2;
   initial s2 = '{int:1, default:0, string:""}; // set all to 0 except the
                                                // array of bits to 1 and
                                                // string to ""
endmodule

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.71%. Comparing base (4221f2f) to head (3bca76f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1049   +/-   ##
=======================================
  Coverage   94.71%   94.71%           
=======================================
  Files         191      191           
  Lines       47656    47656           
=======================================
  Hits        45136    45136           
  Misses       2520     2520           

Files	Coverage Δ
source/ast/types/Type.cpp	95.09% <ø> (ø)

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[MikePopoloski]

Huh, interesting that the grammar doesn't include string as a simple type even though it seems like it makes sense. The only other use of simple_type is inside casting_type which also explicitly adds string to the list, so this change makes sense to me.