Multi-root workspace support

[RichardFurness]

This extension doesn't seem to support multi-root workspaces. If I have a workspace with multiple folders, I find that the Yang LS runs only in one of these folders (the one appearing first in the workspace file). It therefore doesn't parse the Yang modules in the remaining folders (unless I explicitly open them). This leads to errors like failing to find imported modules.

It seems like a fairly simply solution would be to run multiple instances of the LS, one per folder. Then the per-folder yang.settings can be used to specify the include path to handle dependencies between the folders. Of course, this might result in some modules being processed by both instances, so isn't ideal but is better than nothing.

It would be better if it could be handled using a single LS instance, but I can imagine this might require changes to the LS itself (to support multiple roots).

[nckbgov]

To manage such situations
with the actual state of the extension,
I specify external modules in the settings files:

~/.yang/yang.settings

{
"yangPath": "/my/home/yang/libs/rift.zip"
}

More about this:

https://github.com/theia-ide/yang-lsp/blob/master/docs/Settings.md

[gorshkov-leonid]

I have the next use case:
Eclipse Che 7.34.1 uses almost the latest version of Theia (an online editor that implements in browser functionality of vscode). If I may say so, multi-root is required to be supported in LS in Theia to convenient work. For example, I can clone GIT repository and it will be added as a root folder to my workspace by default. How to explain to a user, that it is a prohibited action...

I started searching and found that multi-roots are supported in xtext 2.21.0. But latest version yang-vscode uses yang-lsp that uses xtext 2.20.0 that a priory is not containing multi-root support.

let me refer to developers names here that could say something here:
@JanKoehnlein, @kittaakos, @dhuebner