fix(exrc): exrc knows its own location (#34638)

fix(exrc): lua exrc files know their location

Problem:
'exrc' files are inherently bound to their location / workspace and
therefore require to "know" their location on the filesystem. However,
currently using `debug.getinfo(1, 'S')` returns `"<nvim>"`.

Solution:
Include the filepath as chunkname in `loadstring()` and `nlua_exec()`.

runtime/lua/vim/_meta/options.lua

@@ -2194,6 +2194,8 @@ vim.bo.et = vim.bo.expandtab
 --- Unset 'exrc' to stop further searching of 'exrc' files in parent
 --- directories, similar to `editorconfig.root`.
 ---
+--- To get its own location, Lua exrc files can use `debug.getinfo()`.
+---
 --- Compare 'exrc' to `editorconfig`:
 --- - 'exrc' can execute any code; editorconfig only specifies settings.
 --- - 'exrc' is Nvim-specific; editorconfig works in other editors.