## Summary of the Pull Request
Adds the `target` keybinding arg to `openSettings`. Possible values include: `defaultsFile`, `settingsFile`, and `allFiles`.
## References
#5915 - mini-spec
## PR Checklist
* [x] Closes#2557
* [x] Tests added/passed
## Detailed Description of the Pull Request / Additional comments
Implemented as discussed in the attached spec. A new enum will be added for the SettingsUI when it becomes available.
## Validation Steps Performed
Added the following to my settings.json:
```json
{ "command": "openSettings", "keys":... },
{ "command": { "action": "openSettings" }, "keys":... },
{ "command": { "action": "openSettings", "target": "settingsFile" }, "keys":... },
{ "command": { "action": "openSettings", "target": "defaultsFile" }, "keys":... },
{ "command": { "action": "openSettings", "target": "allFiles" }, "keys":... }
```
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