[vim9script/en] add vim9script (#5215)

* add vim9script as translation of @HiPhish 's legacy vimscript file

* update vim9script using yeggapan's guide

* apply @ubaldot 's kind improvement suggestions

* rearrange content to reduce number of sections

* include most gratefully @kennypete 's improvements

* address @girishji 's suggestion

* add short legacy vimscript intro as suggested by @kennypete

* add initial paragraphs comparing to legacy Vimscript

* re-add paragraph on testing

* shorten intro

Author: Konfekt