Fix extension build warnings

[MaximeKjaer]

Fixes the following warning from yarn build:

 INFO  Detected presense of yarn.lock. Using 'yarn' instead of 'npm' (to override this pass '--no-yarn' on the command line).

See info from vsce --help:

$ npx vsce help package
Usage: vsce package [options]

Packages an extension

Options:
  -o, --out [path]         Output .vsix extension file to [path] location
  --githubBranch [branch]  The GitHub branch used to infer relative links in README.md. Can be overriden by --baseContentUrl and --baseImagesUrl.
  --baseContentUrl [url]   Prepend all relative links in README.md with this url.
  --baseImagesUrl [url]    Prepend all relative image links in README.md with this url.
  --yarn                   Use yarn instead of npm (default inferred from presense of yarn.lock or .yarnrc)
  --no-yarn                Use npm instead of yarn (default inferred from lack of yarn.lock or .yarnrc)
  --ignoreFile [path]      Indicate alternative .vscodeignore
  --noGitHubIssueLinking   Prevent automatic expansion of GitHub-style issue syntax into links
  --web                    Experimental flag to enable publishing web extensions. Note: This is supported only for selected extensions.
  -h, --help               display help for command