[bug] Not working when filetype doesn't match parser name

[artem-nefedov]

In certain cases filetype and parser (language) name can be different, e.g. shell script can have sh filetype and use bash parser.
Currently, vim.treesitter.language.inspect(filetype) check causes failure in such case, e.g. vim.treesitter.language.inspect('sh') throws no parser for 'sh' language error even if bash parser is installed.

This is a regression introduced in 11aea08. Before this commit, it was using vim.treesitter.language.get_lang(), which works properly. For example, vim.treesitter.language.get_lang('sh') will return bash.