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()`.

src/nvim/eval.lua

@@ -2421,7 +2421,8 @@ M.funcs = {
       	<SID>		"<SNR>123_"  where "123" is the
       			current script ID  |<SID>|
       	<script>	sourced script file, or script file
-      			where the current function was defined
+      			where the current function was defined.
+      			Use |debug.getinfo()| in Lua scripts.
       	<stack>		call stack
       	<cword>		word under the cursor
       	<cWORD>		WORD under the cursor