Support XDG locations for files (including plugins)

### Idea

Edit: it seems supporting the XDG spec would satisfy most peoples needs, so renaming to capture this intent.

Currently `asdf-nodejs`, `asdf-ruby` and `asdf-python` among other plugins allow for `$HOME/.default--packages` config files. It is clear from the discussions in each plugin that people want to be able to configure the file location.

### Proposals

Perhaps `asdf` core can provide plugins with `ASDF_CONFIG` dir that is the directory of `.asdfrc` by default?

---

This way, users can configure this, and plugin authors can focus on the default packages filename appropriate for their plugin and install those packages instead of also having to support a diverging set of dirs.

Or perhaps this is too much and simply doing:

```shell
default_gem_path() {
local config_path=${ASDF_CONFIG_FILE:-"$HOME/.asdfrc"}
local config_dir=$(dirname "$config_path")
```

in each plugin is enough. (This was proposed in `asdf-ruby` https://github.com/asdf-vm/asdf-ruby/pull/114/files)

### Related

- https://github.com/asdf-vm/asdf-ruby/issues/111
- https://github.com/asdf-vm/asdf-nodejs/issues/152
- https://github.com/asdf-vm/asdf-ruby/pull/114

Start with the .asdfrc location and ASDF_CONFIG_FILE behavior described in the issue, then compare the default_gem_path example with the related asdf-ruby and asdf-nodejs discussions and pull request. Done should mean that the chosen XDG/config-directory behavior is defined for asdf core and plugins, with the affected plugin configuration locations covered.

Written by the indexing model from the issue text.