Add a license header before getting them in the Vim runtime.

Vim can automatically comment and format comments. Set the necessary
variable to enable that feature.

See `:help format-comments` for more information.

The vim syntax indentation rules stored in indent/meson.vim set the
local shiftwidth and softtabstop variables. As the file is loaded last,
after ~/.vim/after/ftplugin/meson.vim (when present), this prevents
overriding the default values for shiftwidth and softtabstop in a local
configuration.

Fix this by setting shiftwidth and softtabstop in ftplugin/meson.vim
instead (as done by the python indentiation rules in upstream vim for
instance) to allow local overrides.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>