Problem: filetype: ed script files not recognised.
Solution: Add filetype detection for *.ed files and shebang lines,
include syntax script and syntax tests.
Features of the ed syntax file:
- BSD and GNU extensions are supported
- Andrew L. Moore's ed extensions are not supported
- Rebuild synmenu.vim
closes: vim/vim#19602c28515b999
Co-authored-by: Doug Kearns <dougkearns@gmail.com>