Add csharp ls setup for cs filetype (#712)

ftplugin/cs.lua

@@ -0,0 +1,6 @@
+-- C# language server (csharp/OmniSharp) setup
+require("lspconfig").omnisharp.setup{
+  on_attach = require("lsp").common_on_attach,
+  root_dir = require("lspconfig").util.root_pattern(".sln",".git"),
+  cmd = { DATA_PATH .. "/lspinstall/csharp/omnisharp/run", "--languageserver", "--hostPID", tostring(vim.fn.getpid()) },
+}