config: add function to resolve conditional tables

This provides an inline version of Git's includeIf. The idea is that each config
layer has an optional key "--when" to enable the layer, and a layer may have
multiple sub-layer tables "--scope" to inline stuff. Layers can be nested, but
that wouldn't be useful in practice.

I choose "--" prefix to make these meta keys look special (and are placed
earlier in lexicographical order), but I don't have strong opinion about that.
We can use whatever names that are unlikely to conflict with the other config
sections.

resolve() isn't exported as a StackedConfig method because it doesn't make
sense to call resolve() on "resolved" StackedConfig. I'll add a newtype in
CLI layer to distinguish raw config from resolved one. Most consumers will
just see the "resolved" config.

jj-vcs#616

lib/src/config.rs

@@ -33,6 +33,8 @@ use thiserror::Error;
 use toml_edit::DocumentMut;
 use toml_edit::ImDocument;
 
+pub use crate::config_resolver::resolve;
+pub use crate::config_resolver::ConfigResolutionContext;
 use crate::file_util::IoResultExt as _;
 use crate::file_util::PathError;