Add new 'open externally' button

[EdmondChuiHW]

Summary

Add a new button to open the source file at the current line in an external editor , e.g. VS Code

This is current not supported in the community version of Metro or Fusebox.

Test plan

Requires D65551943

1. Button shows up on a source file upon hovering the tab
2. Clicking the button will trigger Metro to open the file
   
3. The file should be opened in the editor with the same line number selected from CDT

Upstreaming plan

• This commit should be sent as a patch to the upstream devtools-frontend repo. I've reviewed the contribution guide.
• This commit is React Native-specific and cannot be upstreamed.

[huntie]

Summarising offline feedback: I think we can improve the UX/discoverability of this, and minimise code inserts, by relocating this button.

[EdmondChuiHW]

Updated design based on @huntie's feedback 🙌

Default:

Custom:

[huntie]

Paired with my other feedback on D65551943.

Suggested change

	var enableReactNativeOpenSourceFilesInExternalEditor: boolean|undefined;
	var reactNativeOpenSourceFilesInExternalEditorButtonBackgroundImage: string|undefined;
	var reactNativeOpenSourceFilesInExternalEditorButtonPadding: string|undefined;
	var enableReactNativeOpenInExternalEditor: boolean|undefined; // Also, deletable if we align /open-stack-frame
	var reactNativeOpenInEditorButtonImage: string|undefined;

[huntie]

Please kill, see D65551943 comment.

[EdmondChuiHW]

Revised:

Default:

Custom:

[huntie]

🚀