Adapt to vim9script's autoload requirements

[bohlender]

The plugin's auto-formatting relies on vim9script which was still "experimental" during development of that functionality. It seems that later iterations of vim9script got a bit more restrictive wrt. naming of symbols so the auto-formatting throws an error on more recent versions of Vim.

Error detected while processing ~/.vim/bundle/vim-smt2/autoload/smt2/formatter.vim:
line   95:
E1263: Cannot use name with # in Vim9 script, use export instead
E117: Unknown function: smt2#formatter#FormatAllParagraphs

Goal: Figure out what exactly the new expectation of vim9script is and make auto-formatting again available for all versions after Vim v8.2.2725.