Implement CSS triggerPropertyValueCompletion setting

[svipas]

When this PR will be merged, I will create a setting inside css-language-features which is in vscode repo.

Related to: microsoft/vscode#68495

[octref]

I'll do some cleanup of the long tesetCompletionFor function later. For now just add another parameter for the setting to expected for testing your changes.

[svipas]

@octref Could you review again? I created tests and made some small changes.

[octref]

Looks great, small nitpick. Thanks!

[svipas]

@octref Let me know if I can create a PR in VS Code to add css.completion.triggerPropertyValueCompletion. But first I need your help to create a description for it 😄

package.json

"css.completion.triggerPropertyValueCompletion": {
	"type": "boolean",
	"scope": "resource",
	"default": true,
	"description": "%css.completion.triggerPropertyValueCompletion.desc%"
},

package.nls.json

"css.completion.triggerPropertyValueCompletion.desc": "TODO: desc",

[octref]

Something like "By default, VS Code triggers property value completion after selecting a CSS property. Use this setting to disable this behavior." would do.