feat(exrc): stop searching in parent directories by unsetting 'exrc'

Problem:
No way for a user to limit 'exrc' search in parent directories (compare
editorconfig.root).

Solution:
A configuration file can unset 'exrc', disabling the search for its
parent directories.

src/nvim/options.lua

@@ -2749,6 +2749,9 @@ local options = {
         directories (ordered upwards), if the files are in the |trust| list.
         Use |:trust| to manage trusted files. See also |vim.secure.read()|.
 
+        Unset 'exrc' to stop further searching of 'exrc' files in parent
+        directories, similar to |editorconfig.root|.
+
         Compare 'exrc' to |editorconfig|:
         - 'exrc' can execute any code; editorconfig only specifies settings.
         - 'exrc' is Nvim-specific; editorconfig works in other editors.