postgresql.conf.sample tab width

I noticed that GitHub is using 4-space tabs for this file [0], presumably
due to this [1] recent change.  I'm not aware of an official policy for
this file, but it seems to be written for 8-space tabs.

One way to fix this is to set our default tab width to 8 in
.gitattributes/.editorconfig (see attached patch).  AFAICT that doesn't
affect source code files.  We could also add a special rule for
postgresql.conf.sample, or we could even convert it to 4-space tabs.  Any
druthers?

[0] https://github.com/postgres/postgres/blob/master/src/backend/utils/misc/postgresql.conf.sample
[1] https://github.blog/changelog/2025-08-07-default-tab-size-changed-from-eight-to-four/

-- 
nathan