Update the minimal version of node/npm used in the repo

[kimadeline]

npm changed the way they handled dependencies in 6.6, and having contributors on npm versions on both sides of it causes dev: true, optional: true clutter in package-lock.json. We can update to anything below npm 6.11 because it breaks webpack (see https://github.com/microsoft/vscode-python/issues/7197).

According to https://nodejs.org/en/download/releases/ we can probably update the .nvmrc in the project to Node.js 10.16.3, which is an LTS version that supports npm 6.9.0.

[kimadeline]

We're now at 12.4:

vscode-python/.nvmrc

Line 1 in 458f545

v12.4.0