chore: revert to node 12

We recently upgraded to node 14 (!289 (merged), !289 (merged)).

We did that because two months ago VS Code updated to node 14.

Unfortunately, I looked into logs and found out that 1% of all requests comes from users who have the latest version of the extension, but they have VS Code that still uses node 12. Releasing the extension with node 14 would break it for the 1% of users.

Since the upgrade is not urgent, I propose doing it in a few months when some of the 1% migrated to the newer VS Code.