Features for hierarchical config distibution

[Vladimir-csp]

This will intersect with #8470, as there needs to be some way of sourcing files dynamically, also ignore missing files.

Hierarchical config distribution would assume loading hypr/hyprland.conf from:

• each item of $XDG_DATA_DIRS (reversed) as low-priority upstream or distribution default configuration
• each item of $XDG_CONFIG_DIRS (reversed) as local admin level config,
• one and only $XDG_CONFIG_HOME as the highest priority.

It can be done either internally in hyprland itself, or in user's config. The latter variant might give users more control.

A static and very barebone version of this can be done with what Hyprland already has, if the default user config would be initiated from a different source and contained this:

# This is a default user-level Hyprland config file.
# Refer to the wiki for more information.
# https://wiki.hyprland.org/Configuring/

# Load upstream configuration
source = /usr/share/hypr/hyprland.conf

# Place override configuration below.
# Or replace this file with the one sourced above and edit it to your liking.

A more sophisticated model would require support for variable expansion and item reversal:

source_multipath keyword and env() token

# This is a default user-level Hyprland config file.
# Refer to the wiki for more information.
# https://wiki.hyprland.org/Configuring/

# Load upstream configuration
source_multipath = env(XDG_DATA_DIRS), :, hypr/hyprland.conf, reverse ignoremissing

# Load local configuration
source_multipath = env(XDG_CONFIG_DIRS), :, hypr/hyprland.conf, reverse ignoremissing

# Place override configuration below.
# Or replace this file with the one sourced above and edit it to your liking.

Explanation:
Syntax of source_mutlipath: path_list, delimiter, subpath, flags

env() token returns value from variable. For example, $XDG_DATA_DIRS usually contains /usr/local/share:/usr/share, $XDG_CONFIG_DIRS might contain /usr/local/etc:/etc or just /etc depending on distro.

Delimiter is ':'

Subpath is hypr/hyprland.conf everywhere.

Flag reverse reverses the path list, to load from /usr/share, then /usr/local/share.

Flag ignoremissing suppresses errors on missing files.

Potentially flag firstfound might be used to source only the first found file instead of all. This can be used for non-overriding config model, without reverse.

To also help with #8470 use case, pipe() token would be useful:

source = env(XDG_CONFIG_HOME)/hypr/pipe(hostname).conf