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/_defaults.lua

@@ -956,7 +956,7 @@ do
         local trusted = vim.secure.read(file) --[[@as string|nil]]
         if trusted then
           if vim.endswith(file, '.lua') then
-            assert(loadstring(trusted))()
+            assert(loadstring(trusted, '@' .. file))()
           else
             vim.api.nvim_exec2(trusted, {})
           end

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.

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