Add workspace folder variable resolution for settings url

[04kash]

This PR adds support for resolving ${workspaceFolder} in the following settings:

• java.format.settings.url
• java.settings.url

If ${workspaceFolder} is present in the provided path, it will be replaced with the absolute path of the workspace folder (retrieved from getRootPaths()). The resolution logic checks all available workspace folders and uses the first matching path that exists.

Fixes: #2529

[rgrunber]

Overall, seems fine. My thinking on the topic has changed a bit since I made the comment at #2529 (comment) but still open to merging this. Specifically, I realized that if the client passes a set of system properties / environment variables to the JDT-LS startup, then those keys will be resolved when used in the form ${...} within a path/uri :

eclipse.jdt.ls/org.eclipse.jdt.ls.core/src/org/eclipse/jdt/ls/core/internal/ResourceUtils.java

Lines 262 to 281 in c2cd627

	StrLookup<String> variableResolver = new StrLookup<>() {
	@Override
	public String lookup(String key) {
	if (key.length() > 0) {
	try {
	String prop = System.getProperty(key);
	if (prop != null) {
	return prop;
	}
	return System.getenv(key);
	} catch (final SecurityException scex) {
	return null;
	}
	}
	return null;
	}
	};
	StrSubstitutor strSubstitutor = new StrSubstitutor(variableResolver);
	return strSubstitutor.replace(path);

. I think I liked this approach because it allows for many different variables to be defined and ultimately the client should know best since it sends the settings to begin with.

Does that help at all with your motivation behind making this PR ?

Also, would you be able to add a testcase ? Probably in JavaSettingsTest is a good place for it.

[rgrunber]

If we take this approach, I would probably put the method to be called at the bottom of ResourceUtils.expandPath(..). That way, it would apply not just to formatter settings, but any attempt to resolve a path that the client has set. I guess you would need to pass in getRootPaths() as well to perform the logic there.

[rgrunber]

I would just return filePath if nothing exists on any of the root paths.

[rgrunber]

I would use WORKSPACE_FOLDER_VAR just to make it a bit more obvious that it's constant.