Add support for drop-in files and `.ignore` files.

[TriMoon]

This extension is becoming better and better ❤️ 👍

It would become even better if it supported drop-in files.
What i mean is that it should auto detect what file type the drop-in extends.
Examples:

• *.service.d/some.conf => Service drop-in
• *.timer.d/some.conf => Timer drop-in.
• *.network.d/some.conf => Network drop-in.
• *.netdev.d/some.conf => NetDev drop-in.
• etc etc...

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Also support for .ignore type files like fe.

• example.service.ignore => Service unit
• example.service.d/some.conf.ignore => Service unit drop-in
• etc etc...

See: systemd.unit(3)

Files (including directories) with names that match certain patterns are generally ignored.
This includes names that start with a "." or end with a ".ignore".

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Currently i use the following, but it is not optimal for obvious reasons. 😉

"settings": {
	"files.associations": {
		"**/*.d/*.conf": "systemd-conf",
		"**/*.d/*.conf.ignore": "systemd-conf",
		"*.service.ignore": "systemd-conf",
		"*.timer.ignore": "systemd-conf"
	}
},

I know these kind of files are already ignored by systemd due to their filename extension, but it would be great if the syntax highlighting and completions for these files keep working using this VSC-Extension 😉
Keep up the great work ! 👍