Canonicalize several details in preparation for upstreaming into vim

Fixes several discrepancies from other built-in vim files to make these files more upstreamable:
  * Move autoindent to indent/vroom.vim so it isn't enabled in the case of `:filetype indent off`.
  * Stop setting tabstop. Vim maintainers like it to be 8 everywhere and it's not worth arguing over since we expand all tabs anyway.
  * Enable line wrapping explicitly with `:set cpo-=C` to support vi-compatible-mode users.
  * Set `b:current_syntax` and add missing source guard.
  * Add standard boilerplate comments documenting various files.