fix: missing client capabilities and markdown descriptions

[sumimakito]

It seems monaco-yaml does not set clientCapabilities while calling getLanguageService, which is exported by yaml-language-server. This causes the doesSupportMarkdown in yaml-language-server always return undefined and makes the markdownDescription not being correctly rendered in the completion item.

However, I can see clientCapabilities is being set by the test helper in yaml-language-server with a constant imported from vscode-json-languageservice.

Similiar usage in jsonWorker in monaco-editor can be found here: https://github.com/microsoft/monaco-editor/blob/f6dc0eb8fce67e57f6036f4769d92c1666cdf546/src/language/json/jsonWorker.ts#L38

This pull request copied the clientCapabilities constant from vscode-json-languageservice to avoid introducing new dependencies.

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[sumimakito]

Replaced by #250

[remcohaszing]

I merged this commit manually.

[sumimakito]

@remcohaszing Cool, thanks. I could not decide which of the two PRs better fits this repo, but your choice helps.