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/lua/executor.h

@@ -26,7 +26,7 @@ typedef struct {
 } nlua_ref_state_t;
 
 #define NLUA_EXEC_STATIC(cstr, arg, mode, arena, err) \
-  nlua_exec(STATIC_CSTR_AS_STRING(cstr), arg, mode, arena, err)
+  nlua_exec(STATIC_CSTR_AS_STRING(cstr), NULL, arg, mode, arena, err)
 
 #define NLUA_CLEAR_REF(x) \
   do { \