standardize vim.log.levels output

[bassamsdata]

Just made a small commit to standardize the output of vim.log.levels.INFO and vim.log.levels.WARN. This change ensures compatibility with mini.notify, which strictly aligns with vim.notify, requiring the level parameter to be of type integer or nil. The explanation of the issue in here #640.

From the help:

vim.notify({msg}, {level}, {opts})                              *vim.notify()*
 {level}  (integer|nil) One of the values from |vim.log.levels|.

Please note that this adjustment doesn't impact other methods like vanilla vim.notify and nvim-notify.

[epwalsh]

Thanks @bassamsdata!