Use shiftwidth() instead of 'shiftwidth' option

[zostay]

Migration of vim-perl#11 by @hoelzro which is a copy of vim-perl/vim-perl#235 and has not been taken care of yet.

Description of issue

The way shiftwidth is used in indent/raku.vim is not the best way. Instead of using sw(), it should use shiftwidth() instead.

This is the patch used by vim-perl:

vim-perl/vim-perl@e42811c

[zostay]

I have not confirmed the behavior itself, but that the problem that was fixed in vim-perl has not been fixed in this plugin.