snippets/go.json: update structure

[zjffun]

Update snippets structure for better compatibility with other extensions.

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[zjffun]

How about update snippets structure for better compatibility with other extensions?

I make an extension to manage snippets and find it difficult to compatible with snippets of this extension, because of the nest structure. zjffun/vscode-snippets-manager#10

I read Snippets in Visual Studio Code and Snippet Guide | Visual Studio Code Extension API but not find nest format of snippets. Then I read source code, find it seems safe to remove .source.go wrap.
https://github.com/microsoft/vscode/blob/ecf479e662923a158c04ab58a835549a312442bf/src/vs/workbench/contrib/snippets/browser/snippetsFile.ts#L242-L248

Thank you.

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