Search global Yarn packages for html-validate as well as global NPM

Grant Ramsayrequested to merge
seapagan/vscode-html-validate:search-yarn-global into master

Check to see if the html-validate package exists in Yarn Global packages; if not, it will check the NPM global as normal. Will prefer the Yarn package if both are installed, since generally, if Yarn is installed, the user will use this over NPM.

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