feat(exrc): search in parent directories (#33889)

feat(exrc): search exrc in parent directories

Problem:
`.nvim.lua` is only loaded from current directory, which is not flexible
when working from a subfolder of the project.

Solution:
Also search parent directories for configuration file.

runtime/doc/vim_diff.txt

@@ -209,6 +209,10 @@ nvim.swapfile:
   swapfile is owned by a running Nvim process. Shows |W325| "Ignoring
   swapfile…" message.
 
+nvim.find_exrc:
+- VimEnter: Extend 'exrc' to also search for project-local configuration files
+  in all parent directories.
+
 ==============================================================================
 New Features                                                   *nvim-features*