Migrate from npm to yarn

[smorimoto]

No description provided.

[avsm]

Why?

[smorimoto]

Yarn is said to be basically faster, and it has better interface. For example, there is no yarn upgrade-interactive --latest like command in npm.

[smorimoto]

Also, for npm, the npm and npx commands are sometimes confusing. But it's often a matter of taste. Note: vscode-ocaml-platform uses yarn.

[avsm]

Looks fine, but needs a rebase against master to merge.

[smorimoto]

Done

[avsm]

thanks!