Mk/Uses/electron.mk: Split fetch- and extract-related codes out to npm.mk

Prefetch and extract functions are not necessarily limited to electron
ports. Those functions can be utilized by any ports which needs an
offline node modules installation.

Previously, non-electron ports wanting to use prefetch and/or extract
functions needs to use USES=electron:env, which is now removed in
favor of USES=npm:<package manager name>.

The following description:
USES= electron:env
USE_ELECTRON= npm:npm-name prefetch extract

is simplified to:
USES= npm:npm-name