Add file types to terraform language server config (#467)

lua/lsp/terraform-ls.lua

@@ -1,4 +1,5 @@
 require'lspconfig'.terraformls.setup{
     cmd = {DATA_PATH .. "/lspinstall/terraform/terraform-ls", "serve"},
-    on_attach = require'lsp'.common_on_attach
+    on_attach = require'lsp'.common_on_attach,
+    filetypes = { "tf", "terraform", "hcl" }
 }