fix(startup): respect $NVIM_APPNAME in $XDG_CONFIG_DIRS paths

Problem:

$NVIM_APPNAME was not respected when searching $XDG_CONFIG_DIRS for
config files. Nvim hardcoded "nvim" when constructing paths like
`$XDG_CONFIG_DIRS/nvim/init.lua`, ignoring the $NVIM_APPNAME environment
variable.

This meant that config files like `$XDG_CONFIG_DIRS/myapp/init.lua` were
not loaded, even though $NVIM_APPNAME was set to "myapp".

Solution:

Use `get_appname()` instead of hardcoded "nvim" for $XDG_CONFIG_DIRS
paths in `do_system_initialization()` and `do_user_initialization()`.
This makes $XDG_CONFIG_DIRS behave consistently with $XDG_CONFIG_HOME,
which already respected $NVIM_APPNAME.

As documented in `runtime/doc/starting.txt` (L1440-L1441):
"In the help wherever `$XDG_CONFIG_…/nvim` is mentioned it is understood
as `$XDG_CONFIG_…/$NVIM_APPNAME`."

See:
https://github.com/neovim/neovim/blob/43339dee40aa7731cc644315c15080e30aa1a67c/runtime/doc/starting.txt#L1440-L1441

Relates to #37405

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