[WIP] extensions: Simplify code used for loading xlets, improve startup speed, fix backtrace uselessness.

[mtwebster]

• Drops 'custom' importer code, using native cjs importer instead.
• Leaves compatibility functions for require() and module.exports

Fixes/improves:

• Faster Cinnamon startup (for my random configuration, ~1550ms down
  to ~1200ms).
• Improved logging.
• More consistent code (use of deprecated features will start getting
  flagged and/or blocked for PRs in Cinnamon and spice repositories.
• Previous code was practically unmaintainable by sane people.
• All stock xlets updated for correctness.

Logging - where before we'd see this useless garbage:

(cinnamon:4334): St-CRITICAL **: 22:19:57.081: st_widget_get_theme_node called on the widget [0x622f6579b3c0 StLabel.hourly-data ("...")] which is not in the stage.
== Stack trace for context 0x622f63105b50 ==
0   7ffd484001c0 b   /usr/share/cinnamon/js/misc/fileUtils.js line 211 > Function:19086 (359f8e6c9c0 @ 279)
1   622f632b5d08 i   /usr/share/cinnamon/js/misc/fileUtils.js line 211 > Function:19059 (359f8e6c970 @ 23)
2   7ffd48400ca0 b   /usr/share/cinnamon/js/misc/fileUtils.js line 211 > Function:18976 (359f8e6c6a0 @ 807)
3   622f632b5b68 i   /usr/share/cinnamon/js/misc/fileUtils.js line 211 > Function:19550 (359f8e6d650 @ 104)
4   622f632b5ab8 i   /usr/share/cinnamon/js/misc/fileUtils.js line 211 > Function:19842 (359f8e6e2e0 @ 681)
5   622f632b5a08 i   self-hosted:1461 (1eb5245b46f0 @ 30)
6   7ffd484015a0 b   self-hosted:852 (359f8e92dd0 @ 15)

we now get:

(cinnamon:4334): St-CRITICAL **: 22:20:36.263: st_widget_get_theme_node called on the widget [0x58cf71ab5180 StLabel.hourly-data ("...")] which is not in the stage.
== Stack trace for context 0x58cf6ee74250 ==
0   7fffae294e10 b   /home/mtwebster/.local/share/cinnamon/applets/weather@mockturtl/3.8/weather-applet.js:19084 (387d8e1fd1a0 @ 279)
1   58cf6eeac1f8 i   /home/mtwebster/.local/share/cinnamon/applets/weather@mockturtl/3.8/weather-applet.js:19057 (387d8e1fd150 @ 23)
2   7fffae2958f0 b   /home/mtwebster/.local/share/cinnamon/applets/weather@mockturtl/3.8/weather-applet.js:18974 (387d8e1fce20 @ 807)
3   58cf6eeac058 i   /home/mtwebster/.local/share/cinnamon/applets/weather@mockturtl/3.8/weather-applet.js:19548 (387d8e1fddd0 @ 104)
4   58cf6eeabfa8 i   /home/mtwebster/.local/share/cinnamon/applets/weather@mockturtl/3.8/weather-applet.js:19840 (387d8e1fea60 @ 681)
5   58cf6eeabef8 i   self-hosted:1461 (2daaca0bf470 @ 30)
6   7fffae2961f0 b   self-hosted:852 (e85adf8c6a0 @ 15)

Requires linuxmint/cjs#136 for xlet 'reload'
functionality to work.