fix: missing client markdown capabilities

[sumimakito]

tl;dr This pull request adds the missing clientCapabilities in the YAML worker to fix the broken support for markdownDescription.

It seems monaco-yaml does not set clientCapabilities upon 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.

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 replaces #248

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[remcohaszing]

Thanks! But why did you go with this approach instead of the one in #248? I actually like that one better at first glance.

[remcohaszing]

I merged #248 instead.