Fix Migration to System.Text.Json

[ferrariofilippo]

Resolved / Related Issues
Items resolved / related issues by this PR.

• Related Bug: Tabs don't restore when 'Continue where you left off' is turned on #10003
• Related Migrate from Newtonsoft.Json to System.Text.Json #9881
• Related Feature: Migrate from Newtonsoft.Json to System.Text.Json #9873
• Related Fixed problem with migration to System.Text.Json #10000

Validation
How did you test these changes?

• Built and ran the app

Comments
Pls leave this open as I'm testing for other bugs/problems related to the migration to System.Text.Json
I apologize for not having tested this deeply before committing

[ferrariofilippo]

I found and fixed 2 problems:

• 1st was related with TabItemArguments (de)serialization
  Details:
  Types needs to be serialized with their FullName in order to work in Deserialization.
  There was a problem with deserialization of object types that were translated to JsonElement.
• 2nd was related with file-operation-conflicts
  Details:
  JsonElement isn't properly deserialized so we have to get its content as string

I've looked for other issues but I found nothing.
Please, let me know if you run into any Json-related problem (bug/performance)

[yaira2]

LGTM