qmlls: implement workspacefolder

Implement workspacefolder functionality in qmlls:

• https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_workspaceFolders
• https://microsoft.github.io/language-server-protocol/specifications/lsp/3.17/specification/#workspace_didChangeWorkspaceFolders

This could work as following:
Create a new class called QQmlCodeModelManager that has all the methods that QQmlCodeModel has. The manager owns a map from "workspace folder path" to QQmlCodeModel, and dispatch the calls to the correct QQmlCodeModel, based on the file URI.

Having different QQmlCodeModels would allow to separate each workspacefolder project into having their own DOM, import paths, build paths, etc.