Copy editor config for the neon extension from PostgreSQL (#8009)

This makes IDEs and github diff format the code the same way as
PostgreSQL sources, which is the style we try to maintain.

pgxn/.dir-locals.el

@@ -0,0 +1,19 @@
+;; see also src/tools/editors/emacs.samples for more complete settings
+
+((c-mode . ((c-basic-offset . 4)
+            (c-file-style . "bsd")
+            (fill-column . 78)
+            (indent-tabs-mode . t)
+            (tab-width . 4)))
+ (nxml-mode . ((fill-column . 78)
+               (indent-tabs-mode . nil)))
+ (perl-mode . ((perl-indent-level . 4)
+               (perl-continued-statement-offset . 2)
+               (perl-continued-brace-offset . -2)
+               (perl-brace-offset . 0)
+               (perl-brace-imaginary-offset . 0)
+               (perl-label-offset . -2)
+               (indent-tabs-mode . t)
+               (tab-width . 4)))
+ (sgml-mode . ((fill-column . 78)
+               (indent-tabs-mode . nil))))

pgxn/.editorconfig

@@ -0,0 +1,14 @@
+root = true
+
+[*.{c,h,l,y,pl,pm}]
+indent_style = tab
+indent_size = tab
+tab_width = 4
+
+[*.{sgml,xml}]
+indent_style = space
+indent_size = 1
+
+[*.xsl]
+indent_style = space
+indent_size = 2