Implement dumpstring command

## Background and Motivation

I have a dump where I can see that 90% of spaces is occupied by strings. When I'm dumping these string I can see

```
> do 00007f6bd64f1d78
Name: System.String
MethodTable: 00007f724f320fa0
EEClass: 00007f724f29a828
Size: 2054610(0x1f59d2) bytes
File: /usr/share/dotnet/shared/Microsoft.NETCore.App/2.1.21/System.Private.CoreLib.dll
String:
Fields:
MT Field Offset Type VT Attr Value Name
00007f724f28b750 40001b0 8 System.Int32 1 instance 1027292 _stringLength
00007f724f288f48 40001b1 c System.Char 1 instance 44 _firstChar
00007f724f320fa0 40001b2 58 System.String 0 shared static Empty
```

But what is the content of the string? Unfortunately, I cannot tell because `dumparray` doesn't work with pointers which `_firstChar` basically is.

## Proposed Feature

`dumpstring | ds` command would be really helpful in this scenario. Or maybeeven `dumpregion` which takes basic address and offset and prints all memory in between. It would cover some advanced cases and string as well. But maybe it worth adding a shortcut for strings anyway since they are so basic