devel/electron39: Update to 39.2.7
While here:
- switch to use USE_ELECTRON features of electron.mk for specifying node package manager, pre-fetching necessary node modules, and extracting the node modules into appropriate directories
- use USES=display for starting/stopping display server on testing
- specify TEST_ENV instead of listing env vars in test target
Changelog:
- https://github.com/electron/electron/releases/tag/v39.2.4
- https://github.com/electron/electron/releases/tag/v39.2.5
- https://github.com/electron/electron/releases/tag/v39.2.6
- https://github.com/electron/electron/releases/tag/v39.2.7
Reported by: GitHub (watch releases)