Fix matches of multiple schemas on "colorScheme" (#15748)

Adds proper `type` for `SchemePair` definition to avoid warnings about matches of multiple schemas.

Same fix as https://github.com/microsoft/terminal/pull/4045

## Validation Steps Performed
- Pointed $schema to local file instead of https://aka.ms/terminal-profiles-schema
- Confirmed warning goes away when using a string
- Confirmed using the light/dark object format still passes validation
- Confirmed values like `"colorScheme": 3` no longer incorrectly pass validation whereas they would before

doc/cascadia/profiles.schema.json

@@ -260,7 +260,8 @@
           "description": "Name of the scheme to use when the app is using dark theme",
           "type": "string"
         }
-      }
+      },
+      "type": "object"
     },
     "FontConfig": {
       "properties": {