fix: Stop using joinpath()

Author: andrewferrier

tests/debugprint.lua

@@ -77,8 +77,9 @@ vim.notify = function(msg, _)
     notify_message = msg
 end
 
-local DATA_PATH = vim.fs.joinpath(vim.fn.stdpath("data"), "debugprint")
-local COUNTER_FILE = vim.fs.joinpath(DATA_PATH, "counter")
+-- FIXME: Switch to joinpath for more elegance once we stop supporting <0.10
+local DATA_PATH = vim.fn.stdpath("data") .. "/debugprint"
+local COUNTER_FILE = DATA_PATH .. "/counter"
 
 local teardown = function(opts)
     opts = vim.tbl_extend("keep", opts or {}, { reset_counter = true })