feat: add option to adjust opacity with Ctrl+Shift+scroll (#19151)

## Summary of the Pull Request
This PR introduces an experimental setting that allows to toggle opacity
changes with scrolling.

## References and Relevant Issues
#3793

## Detailed Description of the Pull Request / Additional comments
By default, holding Ctrl + Shift while scrolling changes the terminal's
opacity. This PR adds an option to disable that behavior.

## Validation Steps Performed
I built the project locally and verified that the new feature works as
intended.

## PR Checklist
- [x] Resolves
https://github.com/microsoft/terminal/issues/3793#issuecomment-3085684640
- [x] Tests ~~added/~~ passed
- [x] Documentation updated
- If checked, please file a pull request on [our docs
repo](https://github.com/MicrosoftDocs/terminal) and link it here:
https://github.com/MicrosoftDocs/terminal/pull/873
- [X] Schema updated (if necessary)

doc/cascadia/profiles.schema.json

@@ -2381,6 +2381,11 @@
           "description": "When set to true, holding the Ctrl key while scrolling will increase or decrease the terminal font size.",
           "type": "boolean"
         },
+        "experimental.scrollToChangeOpacity": {
+          "default": true,
+          "description": "When set to true, holding the Ctrl and Shift keys while scrolling will change the window opacity.",
+          "type": "boolean"
+        },
         "compatibility.allowHeadless": {
           "default": false,
           "description": "When set to true, Windows Terminal will run in the background. This allows globalSummon and quakeMode actions to work even when no windows are open.",