Formatter does not support upper-case fun names

[straight-shoota]

Since #4934 fun names can start with an uppercase letter. Compiling works fine but it seems the formatter is not aware of this change.

Example:

# mylib.cr
lib MyLib
  fun Foo()
end

$ crystal tool format mylib.cr
Error: couldn't format 'mylib.cr', please report a bug including the contents of it: https://github.com/crystal-lang/crystal/issues

expecting IDENT, not `CONST, Foo`, at :2:7 (Exception)