Build/Test Tools: Consider always running npm install with --no-optional

[azaozz]

Currently there are 248 optional third (and forth, fifth) party dependencies in package-lock.json that don't seem needed.

Running npm install --no-optional seems to still install many of them (presumably as they are needed by other dependencies) but reduces the overall size of node_modules by about 40MB and speeds things up a bit.

[azaozz]

Similarly dev-dependencies of third party packages (and sub-dependency packages) don't seem needed either but there isn't a clear way to skip these for the moment (as far as I see). Doing --only=prod would skip top-level (our own) dev dependencies too.

[azaozz]

In 51784.diff: Set npm to skip optional dependencies.

However that may not be good, see the corresponding issue for Gutenberg: ​https://github.com/WordPress/gutenberg/issues/26993.