Roblox XML file syntax highlight support

[violetshine]

I work with Rojo every day. However it is really annoying when VS Code doesn't syntax highlight Roblox place or model files and I have to manually select XML as the language. Roblox files are in XML format so I just added those extensions to XML syntax highlighting extension json

EDIT: This is my first PR ever so if I am doing something wrong, not following contributing guide 100%, something is missing or something like that, please let me know and I will fix that

[alexr00]

Roblox seems pretty specific. @aeschli how specific is too specific for adding file extensions?

@cat-kitten one solution for you would be to add a file association in your settings:

	"files.associations": {
		"*.rbxlx": "xml"
	}

[aeschli]

@alexr00 As the extension is so specific I think we can add it to our defaults, for a good initial user experience.

[tisaconundrum2]

I'm over here reviewing the change log and I see robolox. I couldn't help but come over here to find out the reason behind this. Congrats @cat-kitten on your first merged PR.