Allow to specify additional NBLS document selectors from other extensions.

[dbalek]

Additional document selectors for NBLS can be specified in extension's package.json file as follows:

"contributes": {
	"netbeans.documentSelectors": [
		{
			"language": "yaml",
			"pattern": "**/{application,bootstrap}*.yml"
		},
		{
			"language": "properties",
			"pattern": "**/{application,bootstrap}*.properties"
		}
	]
	...
}