This patch adds the `tree-sitter-config` crate, which manages
tree-sitter's configuration file. This new setup allows different
components to define their own serializable configuration types, instead
of having to create a single monolithic configuration type. But the
configuration itself is still stored in a single JSON file.
Before, the default location for the configuration file was
`~/.tree-sitter/config.json`. This patch updates the default location
to follow the XDG Base Directory spec (or other relevant platform-
specific spec). So on Linux, for instance, the new default location is
`~/.config/tree-sitter/config.json`. We will look in the new location
_first_, and fall back on reading from the legacy location if we can't
find anything.
