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/vimfn.lua

@@ -1871,7 +1871,8 @@ function vim.fn.exp(expr) end
 ---   <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