QML Language Server: Download qmlls instead using it from `qtFolder`

In the current implementation, the extension tries to find the latest version inside `qtFolder`. But this approach is not reliable due to the below cases:

1. `qmlls` might not have the latest features
2. `qmlls` can crash

To fix the above cases, users should install the latest Qt version which is not convenient.

Downloading `qmlls` from a remote repository would remove `qmlls`s dependency on `qtFolder`.

the `clangd` extension is a good example of this feature

https://github.com/clangd/vscode-clangd