refactor(extension): improve code quality

[kiyui]

Refactor code to be cleaner & easier to maintain

Changed proposed:

• Relint with standard
• Rename SliderMenuItem to NightLightSlider
• Refactor NightLightSlider such that NightLightExtension handles view updates and non-slider update on proxy g-properties-changed signal
• Abstract updating night light schedule to a NightLightSchedule class that is in turn handled by NightLightExtension
• Use GLib.timeout_add instead of the deprecated Mainloop function
• Use standard for linting
• Replace makefile build tools with npm

To-do:

• Add standard to package.json and handle with precommit hook
• Handle extension icon instance so can support Change slider when scrolling with the mouse over the battery icon? #17 in the future
• Clean up add to menu logic to support multiple item placements, such as Move slider to "Night Light On" submenu #22
• Add hook to NightLightSlider change event to support Show value on change #18
• Review places where clean-up is necessary on extension disable

Testing:

• Change min & max feature working
• Can status icon feature working
• Show always feature working
• Enable always feature with schedule update is working

[kiyui]

Refactored extension, for testing purposes: [email protected]