Add support for `ts_ls` language server

[AnthonyUtt]

nvim-lsp has updated the name of tsserver to ts_ls for disambiguation purposes. The underlying language server is still the same, so support should be as simple as adding it to the list of supported servers with the same image as tsserver.

I have opened a PR here as well, hope that the contribution isn't out of line. Feel free to let me know I need to make any changes or anything.