Rename phpstan.neon -> phpstan.dist.neon

Per https://phpstan.org/config-reference#config-file:
> The usual practice is to have `phpstan.neon.dist` or `phpstan.dist.neon`
> under version control, and allow the user to override certain settings
> in their environment (on their own computer or on a continuous
> integration server) by creating `phpstan.neon` that’s present in
> `.gitignore` file.

I picked `phpstan.dist.neon` for consistency with
`.php-cs-fixer.dist.php` config (dependabot#5691).

I didn't add the file to `.gitignore` because I actually want to know if
there starts being a new file under that name and see it in git...

composer/helpers/v1/build

@@ -17,7 +17,7 @@ cp -r \
   "$helpers_dir/.php_cs" \
   "$helpers_dir/composer.json" \
   "$helpers_dir/composer.lock" \
-  "$helpers_dir/phpstan.neon" \
+  "$helpers_dir/phpstan.dist.neon" \
   "$install_dir"
 
 cd "$install_dir"

composer/helpers/v2/build

@@ -17,7 +17,7 @@ cp -r \
   "$helpers_dir/.php-cs-fixer.php" \
   "$helpers_dir/composer.json" \
   "$helpers_dir/composer.lock" \
-  "$helpers_dir/phpstan.neon" \
+  "$helpers_dir/phpstan.dist.neon" \
   "$install_dir"
 
 cd "$install_dir"