File detection and syntax highlighting for the QBE compiler backend IL.

• mkdir -p ~/.vim/pack/plugins/start/
• cd ~/.vim/pack/plugins/start/
• git clone https://github.com/perillo/qbe.vim

• Support for indentation (indent/qbe.vim).
• Decide between 4-space or 8-space indentation. The doc/il.txt document seems to suggest 8-space indentation.
• Decide if env is a keyword.
• Verify that the supported string escape sequences are correct.