[dds/dap] Tidy up variables formatting and allow "format specifiers" in evaluation expressions

This adds some new classes to support different formatting of values in expression evaluation/watch/variables panes.

Some existing code that passed a flag around to suppress quotes has been converted to this use, and in addition, we support some C#-inspired "format specifiers" that allow you to influence the formatting of returned values by adding a suffix to a watch/evaluation expression:

https://learn.microsoft.com/en-us/previous-versions/visualstudio/visual-studio-2010/e514eeby(v=vs.100)?redirectedfrom=MSDN

So if you add "foo" and "foo,nq" to your watch window, the latter will format the string without quotes. If you add "a" and "a,h" then the latter will format a in hex.

Format specifiers are carried down, so you can add ",h" to a class or list variable, and each value down the tree will be formatted that way.

Partly fixes https://github.com/Dart-Code/Dart-Code/issues/3940.

Change-Id: I78001bd046ee2586bd828752f9ea08da01e7180c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/279961
Commit-Queue: Ben Konyi <bkonyi@google.com>
Reviewed-by: Ben Konyi <bkonyi@google.com>