Make LSP root detection more robust "borrowing" the lspconfig strategy

[hawk78]

The current implementation does not always get the right path for some LPSs. E.g. lua_ls.

This implementation strategy comes from lspconfig itself, but does not add new any dependency.

The current implementation that this request replaces is using deprecated API in vim.lsp.

[hawk78]

Looks like this project is unmaintained, is anyone here?

[ahmedkhalf]

Not much time to maintain it, will have a look when I get the chance to

[gmerzu]

Hello,

I can confirm this PR targets important issue.

The easiest way to reproduce:

1. Open two files from different projects
2. When switching between the buffers, root dir is the same, since vim.lsp.buf_get_clients()[X].config.root_dir returns the same. As mentioned here, need to follow lspconfig strategy and check for vim.lsp.buf_get_clients()[X].workspace_folders

With this fix, there is no such issue on my side.