VS Code can only load json grammars, so yaml based grammars must be converted to json. The js-yaml package and command-line tool makes this easy.

Install js-yaml as a development only dependency in your extension

$ npm install js-yaml --save-dev

Use the command-line tool to convert the yaml grammar to json

$ npx js-yaml syntaxes/moxi.tmLanguage.yaml > syntaxes/moxi.tmLanguage.json

Build and publish with vsce

https://codevsce package.visualstudio.com/api/working-with-extensions/publishing-extension

Install extension from vsix

https://code.visualstudio.com/docs/editor/extension-marketplace#_install-from-a-vsix