Fix matches of multiple schemas on "profiles" (#4045)

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## Summary of the Pull Request
Adds proper `type` for `ProfilesObject` definition to avoid warnings about matches of multiple schemas.

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## References
Original issue: #3909
Related PR: #3892
Relates VSCode issue: microsoft/vscode#86738

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## PR Checklist
* [X] Closes #3909
* [X] CLA signed. If not, go over [here](https://cla.opensource.microsoft.com/microsoft/Terminal) and sign the CLA
* [ ] No new tests ~Tests added/passed~
* [ ] No docs update needed ~Requires documentation to be updated~
* [X] I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #3909 (marked as help wanted)

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## Detailed Description of the Pull Request / Additional comments

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## Validation Steps Performed
1. Download `doc/cascadia/profiles.schema.json` locally
1. Open `profiles.json` from WT in VSCode
1. Replace `$schema` value with path to local copy (verified that all errors are still in place and validations works as before)
1. Update it with `type` on `ProfilesObject`
1. Check that `Matches multiple schemas when only one must validate` warning is fixed

doc/cascadia/profiles.schema.json

@@ -622,7 +622,8 @@
           "description": "The default settings that apply to every profile.",
           "$ref": "#/definitions/Profile"
         }
-      }
+      },
+      "type": "object"
     },
     "SchemeList": {
       "description": "Properties are specific to each color scheme. ColorTool is a great tool you can use to create and explore new color schemes. All colors use hex color format.",