refactor(cli): move get_language out of File_To_Lint

get_language is a bit weird. There are two functions: a top-level
function and a member function of File_To_Lint. Make both versions
top-level functions. This makes it more obvious that the functions do
the same thing, and will make it more natural to add some features to
get_language later.

src/quick-lint-js/cli/main.cpp

@@ -301,7 +301,7 @@ void run(Options o) {
         source.error().print_and_exit();
       }
       Linter_Options lint_options =
-          get_linter_options_from_language(file.get_language());
+          get_linter_options_from_language(get_language(file));
       lint_options.print_parser_visits = o.print_parser_visits;
       reporter->set_source(&*source, file);
       parse_and_lint(&*source, *reporter->get(), config->globals(),

src/quick-lint-js/cli/options.cpp

@@ -261,8 +261,8 @@ bool Options::dump_errors(Output_Stream& out) const {
   return have_errors;
 }
 
-Resolved_Input_File_Language File_To_Lint::get_language() const {
-  return quick_lint_js::get_language(this->path, this->language);
+Resolved_Input_File_Language get_language(const File_To_Lint& file) {
+  return get_language(file.path, file.language);
 }
 
 Resolved_Input_File_Language get_language(const char* file,

src/quick-lint-js/cli/options.h

@@ -53,10 +53,9 @@ struct File_To_Lint {
 
   bool is_stdin = false;
   std::optional<int> vim_bufnr;
-
-  Resolved_Input_File_Language get_language() const;
 };
 
+Resolved_Input_File_Language get_language(const File_To_Lint &file);
 Resolved_Input_File_Language get_language(const char *file,
                                           Raw_Input_File_Language language);