Use new parser from vs-solutionpersistence to later enable .slnx support

[Meera Ruxmohan (mruxmohan4)]

This change performs the following:

• Replaces custom solution file parsing with the new parser from vs-solutionpersistence library
• Removes hardcoded headers, in addition to global and project sections
• Depends on a new solution serializer which saves the solution model to the solution file
• Adds relevant solution properties to the solution model (to match old behavior)

From initial local testing:

• No differences in SolutionConfigurationPlatforms global section
• Only differences in ProjectConfigurationPlatforms global section pertained to project guids (however, order remained the same), which is expected behavior
• No differences in the SolutionItems project section
• No differences in SolutionProperties and ExtensibilityGlobals, apart from the solution guid, which is recreated upon each slngen invocation
• Only differences in the set of initial projects defined in the solution file pertained to project guids (however, order remained the same), which is expected behavior

[kasperk81]

slngen's command line options are

  -d|--solutiondir <path>             An optional path to the directory in which the solution file will be generated. Defaults to the same directory as the project. --solutionfile will take precedence over this switch.
  -o|--solutionfile <path>            An optional path to the solution file to generate. Defaults to the same directory as the project.

consider adding --format <sln|slnx>

upcoming dotnet SDK 9.0.2xx (and v10 from daily builds) has

dotnet new sln --help
...
Template options:
  -f, --format <sln|slnx>  Choose the format for the solution file: sln or slnx.
                           Type: choice
                             sln   Solution file
                             slnx  XML Solution file
                           Default: sln

see the feedback from VS team about naming/terminology dotnet/sdk#44469 (comment)

[kasperk81]

Meera Ruxmohan (@mruxmohan4) any plans to revive this?