Updates `Config::tryExpandPath()` to return a vector of expanded path
matches instead of a single path wrapped in an optional, with an empty
vector indicating no matches.
`Config::resolveConfigIncludes()` iterates over all of these matches,
while other instances of path expansion (such as finding the base config
path) retain their existing behavior and only use the first match.
This commit adds support to reading the config base path from the
environment variable `WAYBAR_CONFIG_DIR`. If it is set, but no
configuration is found there, it falls back to the previous mechanism
of using the default paths, without erroring.
